Method and system for mapping and tracking information from a plurality of remote stations

ABSTRACT

A method of tracking changes in position of a plurality of remote stations, each having, the method comprising:  
     (a) assigning a plurality of transmission slots to each of the remote stations;  
     (b) determining, by the respective stations, of their positions;  
     (c) initially transmitting, by the respective stations of their positions;  
     (d) determining, by the respective stations, of their positions, relative to the previously determined and transmitted positions; and  
     (e) subsequently transmitting, by the respective stations, of the determined relative positions,  
     wherein the slots themselves indicate the relative determined position and wherein the determined relative position is indicated by the presence or absence of a signal in one or more of said plurality of slots.

RELATED APPLICATIONS

[0001] This application is a continuation in part of PCT applicationPCT/EP95/01330, filed Apr. 10, 1995, which designates the United Stateswhich is a continuation in part of U.S. patent application Ser. No.08/232,776, filed Apr. 25, 1994, which is a continuation in part of PCTapplication PCT/EP93/03418, filed Dec. 6, 1993 which designates theUnited States.

FIELD OF THE INVENTION

[0002] This invention relates generally to a method and system forobtaining information from a plurality of remote stations.

BACKGROUND OF THE INVENTION

[0003] It is a common requirement to target and possibly identifyquickly one or several out of a plurality of participants according tospecific selection criteria.

[0004] It is frequently required to select one or more participantsaccording to a “priority” or “characteristic value” based on specifiedselection criteria for the purpose of allocating a particular task tothe participant or participants having the highest priority.

[0005] In dispatching systems, for example, for dispatching a taxi ormessenger to a customer at a specified location, it is desirable that asuitable (and preferably the most suitable) taxi or messenger be sent toa particular customer. Generally the nearest, unoccupied taxi which hassufficient accommodation should be dispatched to the customer.Furthermore, it is desirable that the allocation be accomplished in theminimum possible time.

[0006] Typical existing dispatching systems include a central dispatchstation having a transmitter and receiver or a transceiver in each ofthe participating vehicles for communicating with the central dispatchstation. Typically, a voice request is transmitted by a dispatcher toeach of the participating vehicles, and the dispatcher decides which ofthe vehicles is most suited to the task in hand based on the repliesfrom the vehicles.

[0007] Such a system would be capable of simple implementation if theselection criteria related to static variables only. Thus, if the onlyselection criterion were a taxi's current distance from the customer andeach taxi were stationary, it would merely be necessary to extract thetaxis' locations once, after which it would be simple to determine whichtaxi were nearest to the customer's location. However, in practice, theselection criteria relate to dynamic variables which, by definition, arechanging constantly and therefore it is necessary continuously to updateeach taxi's distance from the customer's location (and/or otherinformation required to choose a taxi for the given task) or at least todo so each time a taxi is to be dispatched.

[0008] In some typical prior art systems, this is done by providing thedispatcher with a periodically updated map that shows the respectivelocation of each of the taxis. This updating is accomplished by theperiodic transmission of a location message by each of the taxis via acommunication channel. In order to ensure that the transmitted data canbe received quickly and without corruption, the total spectrum width ofthe communication system must be very large.

[0009] In a system described in EP 0389488 job requests are dispatchedby a controller to mobile vehicles which messages include informationabout the location of a job. Each vehicle has a receiver, transmitterand circuitry to compare the requirements of the job with the status ofthe vehicle. If the results of the comparison is that the vehicle issuitable for the job, then it transmits a message back to the controllervolunteering itself for the job.

[0010] It should also be noted that, even in the specific case of a taxior messenger service, distance from the customer location is by no meansthe only criterion according to which a task may be allocated. Thus, itmay well be that the nearest messenger or taxi is already occupied andis therefore not available for performing the task. Alternatively, thenearest available taxi may not have sufficient room for carrying all thepassengers to whom a taxi must be sent; or perhaps a particularly bulkyload must be carried and the nearest, available taxi or messenger isinadequate for the task.

[0011] Yet a further consideration is that it is often preferred todispatch to a customer an idle taxi waiting at the taxi rank rather thango through the process of transmitting a voice message and awaitingresponses from taxis in the field prior to allocating the task to one ofthem. In the event that several idle taxis are waiting at the taxi rank,or where several taxis are reasonably close to the customer, it is oftenpreferable that the taxi which has been idle for the longest period oftime be selected.

[0012] Furthermore, it may not always be desirable to dispatch thenearest available taxi to a particular customer location if othercustomers, albeit further away, have made prior requests which have notyet been serviced.

[0013] Even apart from some of the basic limitations of prior artsystems described above, it is often desirable to target and possibly toidentify participants according to several selection criteria. This issomewhat analogous to performing a database search by means of key wordswhich can be combined according to the rules of Boolean or other logicsystems. However, database records are generally static and are storedat a single location. In contrast to this, the attributes of theparticipants that are the subject of the present invention are dynamicand constantly changing, and cannot be characterized by static datawhich can be stored at a single site. Thus, if the dynamic datacharacterizing such participants are to be searched at a single site,then the data must first be downloaded to the site where the search isto be performed. During the time that such data are downloaded, they maywell change, thereby compromising the accuracy of the search which issubsequently performed.

[0014] Another application which requires the receiving and processingof information from a large number of sources is IVHS. In thisapplication, for example, information on position and speed from a largenumber of vehicles is processed in order to obtain information on roaddelays. Again, the sending of large amounts of information requiressubstantial bandwidths, even though the vehicles themselves need not beidentified.

[0015] Another previously unsolved problem is the tracking or mapping ofthe position of large numbers of vehicles. Prior solutions to thisproblem required the broadcasting by each vehicle of an informationbearing signal including at least its position. When large numbers ofvehicles are to be tracked, the amount of information to be transmitted(and the communication overhead associated with the transmission) isvery large and the available time/bandwidth necessary is eitherunavailable or if it is available, such broadband systems are expensive.The alternative of trading bandwidth for time, results in a system whichis too slow for many uses.

SUMMARY OF THE INVENTION

[0016] It is an object of some aspects of the present invention totransmit information from a plurality of remote sources withoutrequiring that each of the transmissions be on a separate time/frequencychannel.

[0017] It is an object of some aspects of the invention to provide amethod of transmitting information from a plurality of remote stationswherein the information is contained in the presence or absence of asignal in a particular time and/or frequency slot and not in theidentification of the particular station which transmits theinformation, in the transmission of an information bearing signal by theremote station and/or in how many stations transmit signals in the slot.

[0018] It is an object of some aspects of the invention to provide amethod and system for determining “priorities” or “characteristicvalues” of a plurality of participants in accordance with one or moreselection criteria and targeting those participants, if any, having thehighest priority or the most suitable characteristic value.

[0019] It is a further object of some aspects of the invention toprovide such a method and system wherein those participants having thehighest priority or the most suitable characteristic value can betargeted in a short time.

[0020] Yet a further object of some aspects of the invention is toprovide such a method and system wherein at least one of the targetedparticipants can be identified in order that a task can be allocatedthereto.

[0021] Yet another object of some aspects of the invention is to providean improved system for real time bus routing.

[0022] It is an object of some aspects of the present invention toprovide a method of determining traffic delays in a road network basedon transmissions from a large number of vehicles without identifying thevehicles and without receiving information associateable with aparticular vehicle.

[0023] It is an object of some aspects of the present invention toprovide a system for almost real time mapping of the positions of largenumbers of moving stations with higher accuracy and with greater speedthan prior art systems.

[0024] As used herein the term “priority” or “characteristic value”means, in addition to its normal meaning, a characterization accordingto a protocol which takes into consideration one or more elementsassociated with a person or object being characterized.

[0025] According to a broad aspect of the invention a call is broadcastor otherwise transmitted to a plurality of remote stations. Each of theplurality of stations determines a characteristic value or prioritybased on certain predetermined attributes of the station and broadcastsor transmits an indication signal during a communication slot which isindicative of the characteristic value, or preferably of a range of thecharacteristic value. In some aspects of the invention, more than oneremote station will broadcast at the same time and frequency during atleast a portion of the process.

[0026] In one embodiment of the invention, all of the stations broadcastor transmit at the same frequency, i.e. all of the slots have the samefrequency and the time of the slot is determined by the range ofcharacteristic values. In a second embodiment of the invention, morethan one frequency is used for communication and both the time andfrequency indicate the characteristic value. In a third embodiment ofthe invention, all of the stations transmit at the same time, and thecharacteristic value is indicated by the frequency of transmission only.

[0027] It should be understood that since more than one indicationsignal may be broadcast or transmitted at the same time and frequency,there is no identification of the responding stations, but only anindication of the characteristics (or rather ranges of characteristics,since each time/frequency “slot” generally represent a range ofcharacteristic values) which characterizes at least one remoteresponding station.

[0028] It should also be understood that in many preferred embodimentsof the invention, the transmitted information signals are“non-information bearing signals” in that the signals per se carry noinformation, only the slot in which the signal occurs carriesinformation. Information about the identity of the transmuting stationmay not be of present interest or alternatively, in some applications,certain slots are used only by a given remote station, whereby thebroadcasting station may be identified.

[0029] According to one aspect of the invention the characteristics areone or more priorities associated with the stations.

[0030] In one aspect of the invention a control center monitors thetransmissions of the remote stations and determines which of the slotshaving an indication signal has the highest priority. It is convenientto order the response time period into time (or time/frequency) slotseach representing a range of priority values preferably in descendingorder of priorities. Thus, the control center need only look for theearliest slot which contains an indication signal.

[0031] Having determined the highest range of priorities which are heldby at least one remote station, a second call is preferably broadcast orotherwise transmitted asking for responses only from those remote unitswithin this range. The time or time/frequency slots are now distributed,either by a predetermined protocol or specifically by the particularcall, so as to cover this range of priority values.

[0032] The stations which have priorities within this range broadcast orotherwise transmit indication signals in response to the new call in thepredetermined time or time/frequency slots. This process of determiningthe highest range of priorities and redividing the range continues untila given criteria is met. This stage of the process often termed hereinthe “targeting phase,” (sometimes referred to herein as the “firstphase” or “phase one”) ends when the priority range ceases to besignificant or the number of sub-ranges which are filled falls below apredetermined number based on the statistics of the total number ofparticipating remote stations and the final range or priorities or wheresome other predetermined criteria is reached. At this point the numberof station which are responding to the highest priority is believed tobe small.

[0033] Before going on to the next stage it may be useful to estimatethe number of stations which have responded to the highest priority. Onemethod of making the estimate is by analysis of the data from the finalstep of phase one. A more accurate method of estimating the number ofstations having the highest priority is to request each of thesestations to transmit an indication signal at a randomly chosen slot overat least a portion of the entire range of time and frequency slots.Since the number of slots is now expected to be large compared to thenumber of stations, the number of slots which have signals is a goodindication of the number of stations. An estimate of the actual numberof responders is then based on the statistical relationship between theactual number of responders and the number of slots in which a signal isbroadcast. If fewer stations are expected, only a single time slot maybe used and only the frequency is chosen randomly by the stations.

[0034] The system then preferably initiates an “identification phase”(sometimes referred to herein as “second phase” or “phase two”) startingwith the broadcast or other transmission of an additional callrequesting those stations within the highest (final) range of prioritiesfound in the targeting phase to identify themselves. Each of thestations having a priority in this range broadcasts or otherwisetransmits a signal including an identifier of the station or some othermessage at a slot which it chooses, preferably at random, from one of aplurality of such available slots. If only one station is expected to bewithin the range of priority values, then only one identification slotmay be allocated. Other types of slots can also be used for theidentification stage, such as coded spread spectrum signals, FDMA orCDMA. Additionally, multiple slots may be used for the same priorityrange to improve the reliability of detection in both the targeting andidentification stages. The identification slots generally have aninformation carrying capacity which is larger than the slots used forindicating priorities since information (and not only an indication ofthe presence of a signal) is transmitted during the identificationphase.

[0035] Since, when a plurality of remote stations are within the finalrange of values, it can be expected that more than one of the stationswill respond in at least some of the slots, in which case theiridentification signal may be unintelligible. However, since the numberof stations is relatively small, at least some of the slots will haveonly one identification signal. In general, the station having thissignal is chosen since at this stage the difference in priority betweenthe stations is generally unimportant. In some applications more thanone identification signal may be broadcast in a particular slot,however, one signal may be clear. This station will then be chosen.

[0036] Alternatively, the stations which are left at this stage may beidentified by assigning to each of the remote stations (including thosewhich are no longer left, since the system has no indication of thosewhich are left) a slot which is associated with only one remote station.All of the stations which are left after the previous stage are invitedto broadcast in their identification slots. One or more of theseresponding stations is then chosen. Using this system of identificationof the remote stations frees them of the need to transmit anyinformation bearing signal, simplifying the system.

[0037] In another aspect of the invention the indication signal dependson the average velocity or delay of the remote station, which aregenerally vehicles. Systems which operate according to this aspect ofthe invention preferably broadcast a call to the remote stations whichrequests those stations having a delay above a given value or an averagevelocity below a given value to broadcast a signal indicative of theirposition. Such signals are then used to generate a map of those regionsfor which traffic is delayed or otherwise moving slowly.

[0038] Preferably, an additional call is sent to the vehicles requestingtransmission of indication signals which position the slow moving ordelayed vehicles at a higher resolution than that of the first call.Further calls may be made to allow for transmission of additionalinformation on the status of the vehicles to provide furthercharacterization of the delays.

[0039] In a preferred embodiment of the invention optimum routing ofbuses is made based on their positions along the route. In accordancewith this embodiment, information on bus positions along the bus line istransmitted to a central dispatch station which calculates a newoptimized schedule based on these updated situation reports. It shouldbe understood that, while in general position is a two dimensionalvector, the position of the bus along its route can be given by a singlevariable.

[0040] In a further preferred embodiment of the invention the positionof a large number of vehicles can be mapped and tracked in near realtime using a relatively narrow bandwidth. In this embodiment eachvehicle is assigned a number of slots which are used only by thatvehicle.

[0041] The vehicles must first be mapped in a preferred first, mapping,phase of the mapping and tracking procedure. In the first step of thisphase, the total area of interest is divided into preferably nine areas,each of which is assigned a slot. The vehicle broadcasts a signal in theslot which corresponds to its present position. In a second step of themapping phase, the area previously indicated as containing the vehicleis expanded to fill the nine slots. Alternatively, the area which iszoomed into the nine slots is slightly larger than the area of theprevious broadcast to avoid a situation in which the vehicle was at theborder of the area and left the area between steps.

[0042] This identification of one area and consequent new zooming andsub-division is repeated several times until the required resolution isachieved. The highest practical resolution, as will become clear below,is the distance that a vehicle could travel in the time it takes toperform a tracking cycle as described below. Within five iterations theindividual resolution can be improved from 3.3 km to about 40 m.

[0043] In a, second, tracking phase of the mapping and trackingprocedure, performed periodically after the required resolution isreached, preferably, nine slots, representing a 3×3 area of resolutionareas, are used to track additional movements of the vehicle. Thecentral one of the nine areas corresponds to the area occupied by thevehicle at the end of the mapping phase (or during a previous periodicupdating iteration of the tracking phase). During each periodic update,each vehicle broadcasts in a slot which corresponds to either itsprevious position (the slot corresponding to the center area of the 3×3group of areas) or one of the adjoining areas. In the next followingiteration, the newly chosen area is the center of the 3×3 matrix.

[0044] In a further preferred variation of this embodiment of theinvention, only 5 slots are utilized to map into the 3×3 area. One ofthese slots represents one of the corners (or the center) of the 3×3area and the other 4 slots represent north south or east westvariations.

[0045] In a further preferred embodiment of the invention, nine areasare represented by a four bit word which is sufficient to define the 3×3matrix of elements.

[0046] In general, one or more base stations may be used forbroadcasting calls and/or receiving responses from remote stations. Ifmore than one base station is used, each station preferably performs areduction of the data which it receives by either choosing its bestcandidate for performing the task or by performing a mapping function ofits nearby region or of its associated vehicles. The base stations thenpreferably send this reduced information to a central base station whichmakes the final decision, constructs the desired map or performs anyother final analysis. Furthermore, the central base station would, in apreferred embodiment of the invention, instruct each of the basestations as to which additional queries they should make. In thissituation the subsequent queries need not be the same for all the basestations.

[0047] There is therefore provided, in accordance with a preferredembodiment of the invention, a method mapping of the characteristicvalues of a plurality of remote stations each having a varying attributeaffecting a characteristic value computed according to a predeterminedprocedure comprising:

[0048] (a) assigning a plurality of transmission slots to each of theremote stations;

[0049] (b) determining, by the respective stations, of theircharacteristic values;

[0050] (c) initially broadcasting, by the respective stations, of theirdetermined characteristic values in said plurality of transmissionslots, said broadcast characteristic value having a first characteristicvalue resolution; and

[0051] (d) subsequently broadcasting, by the stations, of theirrespective characteristic values in said plurality of transmissionslots, said subsequent broadcasting having a finer characteristic valueresolution relative to said previously broadcasted characteristic valuehaving a first characteristic value resolution.

[0052] Preferably, (d) is repeated with successively highercharacteristic value resolution until the characteristic value isbroadcast with a characteristic value resolution. The higher resolutionpreferably twice, or somewhat less than twice the first positionresolution.

[0053] In a preferred embodiment of the invention, a mapping space isdivided into a fixed number of portions and wherein said initialbroadcast indicates which of said portions contains the position.Preferably, the initially broadcast portion or a portion somewhat largerthan the initially broadcast portion is divided into a fixed number ofportions of a smaller size and wherein said subsequent broadcastindicates which of said portions of smaller size contains thecharacteristic value.

[0054] There is further provided in accordance with a preferredembodiment of the invention, a method of tracking a characteristic valueof a plurality of remote stations each having a varying attributeaffecting the characteristic value computed according to a predeterminedprocedure, comprising:

[0055] (a) assigning a plurality of transmission slots to each of theremote stations;

[0056] (b) determining, by the respective stations, of theircharacteristic values relative to a previously determined characteristicvalue; and

[0057] (c) broadcasting, by the respective stations, of their determinedcharacteristic values in said plurality of transmission slots, relativeto the previously determined characteristic value.

[0058] Preferably the method includes iteratively repeating (b) and (c)wherein said previously determined characteristic value is thecharacteristic value determined in the previous iteration. Preferably, acharacteristic value region surrounding said previously determinedcharacteristic value is divided into a plurality of contiguous regionsand wherein the relative characteristic value which is broadcastcomprises broadcasting a signal in one or more of the transmission slotswhich indicates which of the regions contains the determinedcharacteristic value. More preferably, the extent of the surroundingregions is established based on an expected maximum rate of change ofthe characteristic value in the remote station.

[0059] In a preferred embodiment of the invention, the previouslydetermined characteristic value is determined in accordance with themapping method.

[0060] A preferred embodiment of the invention includes repeating atleast one step of broadcasting, at a coarser characteristic valueresolution, when a valid signal is not received from a remote stationduring mapping or tracking.

[0061] In a preferred embodiment of the invention at least one step ofbroadcasting is repeated periodically to avoid accumulated errors intracking or mapping.

[0062] In an especially useful preferred embodiment of the invention,the characteristic value is the location of a mobile remote station.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] In order to better understand the invention and to see how thesame may be carried out in practice, non-limiting preferred embodimentsof the invention will now be described with reference to theaccompanying drawings, in which:

[0064]FIG. 1 shows schematically the principal components of a preferredsystem for carrying out a dispatching function in accordance with onepreferred embodiment of the invention;

[0065]FIG. 2 is a flow diagram showing the principal steps associatedwith a preferred method of carrying out a dispatching function inaccordance with the preferred embodiment of the invention;

[0066]FIG. 3 shows schematically how vehicles are targeted during aninitial phase of targeting based on distance from the customer locationin accordance with a preferred embodiment of the invention for carryingout a dispatching function;

[0067]FIG. 4 shows schematically how vehicles are targeted during asecond iteration of targeting based on distance;

[0068]FIGS. 5A and 5B are two portions of a state diagram showingvarious options associated with a first targeting phase according to apreferred embodiment of the invention for carrying out a dispatchingfunction;

[0069]FIGS. 6A and 6B are two portions of a state diagram showingvarious options associated with a second identification phase accordingto a preferred embodiment of the invention for carrying out adispatching function;

[0070] FIGS. 7A, and 7B show timing diagrams relating to the targetingand identification phases respectively of a priority discriminationaccording to a preferred embodiment of the invention for carrying out adispatching function;

[0071]FIG. 8 is a block diagram showing the principal components in acontrol center according to a preferred embodiment of the invention forcarrying out a dispatching function;

[0072]FIG. 9 is a block diagram showing the principal components of acontrol unit in respect of each of the remote units in accordance with apreferred embodiment of the invention for carrying out a dispatchingfunction;

[0073]FIG. 10 shows an initial map generated in an IVHS system inaccordance with a preferred embodiment of the invention;

[0074]FIG. 11 shows a second, more detailed map, generated during asecond iteration in an IVHS application in accordance with a preferredembodiment of the invention;

[0075]FIG. 12 shows a graph of additional information which is generatedin an IVHS application in accordance with a preferred embodiment of theinvention;

[0076]FIG. 13 shows a graph of further additional information which isgenerated in an IVHS application in accordance with a preferredembodiment of the invention;

[0077]FIG. 14 is a general block diagram of a transmitter for an IVHSsystem in accordance with a preferred embodiment of the invention;

[0078]FIG. 15 is a block diagram of a receiver for useful for both IVHSand dispatching systems in accordance with a preferred embodiment of theinvention; and

[0079] FIGS. 16A-16C shows a scheme for slot distribution for trackingof individual vehicles.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0080]FIG. 1 shows a typical scenario of a dispatch situation inconnection with which the invention may be employed. In this scenario ageographical area 10 is defined by a boundary 11 within which a systemaccording to the invention is operational.

[0081] An identification system according to the invention includes acontroller such as a control center (base station) 12 and, optionally, aplurality of taxi stands 13, 14 and 15 which constitute sub-controlunits each of which serves a respective region within area 10 and whichmay forward a customer request to control center 12.

[0082] Associated with each of stands 13, 14 and 15 are respectivegroups of participants (e.g. taxis) of which two groups 18 and 19associated with stands 13 and 15, respectively, are shown in FIG. 1. Thegroups of participants 18 and 19 generally comprise some taxis which arestationary proximate their respective stands awaiting instructionstherefrom and other taxis such as 20, 21, 22, 23 and 24 which arecirculating within area 10 and are either available for performing atask on receiving instructions or, alternatively, are occupied andtherefore unavailable.

[0083] A customer 25 located somewhere within area 10 relays a requestfor service to control center 12 telephonically via a Public SwitchedTelephone Network (PSTN). Control center 12, in turn, broadcasts aninvitation message to all of the participants in area 10 either directlyor via a remote station 30 which is typically located so as to cover allof area 10. Control center 12 can also receive messages via station 30.Alternatively, control center 12 broadcasts and receives messagesdirectly.

[0084] Remote station 30 may be located inside area 10, or alternativelyif the area is built up with tall buildings, outside the boundary of thebuilt-up area, if this siting reduces the blocking of signals betweenthe taxis and the repeater station by tall building and the like or forother reasons.

[0085] Sometimes, customer 25 telephones a particular taxi stand sincethis is the nearest stand to the customer's location. In this case it isgenerally preferable that one of the taxis associated with the taxistand be dispatched to the customer unless, of course, all of the taxisassociated with the stand are currently occupied (or otherwiseunsuitable), in which case one of the taxis associated with another ofthe stands will be allocated for the task.

[0086] In this case, the association of a taxi with a particular standmay constitute at least one of the criteria involved in choosing thetaxi to be allocated to customer 25. Such a selection criterion is astatic variable and, once fixed, never varies because a taxi is alwaysassociated with one stand. However, the actual priority assigned to eachof the taxis is also a function of several independent dynamic factorswhich are subject to constant fluctuation. Of these, the taxi's distancefrom the customer is the most important example. However, other dynamicconditions pertaining to a taxi's instantaneous status also affect therespective priority of the taxi so that, for example, a taxi which iscurrently occupied or one which has insufficient occupancy for thenumber of passengers to be collected would not participate in theselection process and a taxi which is waiting at a stand would getpriority. The idle time of the taxi can also be an important criteria.

[0087] It will be appreciated that in general there are many differentcontributory factors, or selection criteria, which influence thepriority assigned to each individual taxi within area 10. Moreover, itis generally the case that each selection criterion has a different“weight” associated therewith so that the final magnitude of thepriority associated with each respective taxi is built up from manydifferent selection criteria each of which exert a different influenceon the actual priority assigned.

[0088] For example, in the simplest case, it may be that only distancefrom the taxi to customer 25 is of concern. Such a simple case would nottake into consideration the fact that other customers may already haverequested service and may not yet have been processed. Thus, anothercustomer near customer 25 but still somewhat further away from thenearest available taxi in area 10, may have a prior claim for service.However, in the simplest of systems where only distance from the taxi tothe customer is important, such a prior claim would not be recognized.

[0089] In a preferred system where many factors are taken into accountthe priority assigned to each taxi may often be viewed as amulti-dimensional vector which is the vector sum of component priorityvectors each relating to a different selection criterion.

[0090] A preferred method for allocating a task to one of the taxis inresponse to a request by customer 25 will now be explained withreference to FIGS. 2 to 7. For the sake of simplicity of presentationonly, it will initially be assumed that the only selection criterion ofinterest is the distance of a taxi from customer 25.

[0091]FIG. 2, shows a flow diagram of the operation of a preferredsystem of the invention. The left portion of the flow diagram comprisesoperation of a first, targeting, phase and the right portion of the flowdiagram comprises operation of a second, identification phase. Thetargeting phase starts with the broadcasting of a call message to all ofthe participating taxis informing them that a priority must bedetermined for responding to a pending request for service. The criteriafor determining the priority may be sent together with the call or,alternatively, may be part of a preset protocol used for all suchdeterminations. Alternatively, there may be several such protocols oneof which the call identifies by a code. In the very simple case of adispatching system wherein distance of a taxi from a specified locationis the only selection criterion, there is no need to inform the taxi ofthe selection criterion each time an invitation message is transmitted.

[0092] Responsive to the call, each of the taxis uses the selectioncriteria to determine its own priority in accordance with the protocol.The protocol also includes a plurality of ranges of priority values anda communication protocol which subdivides a time period and/or afrequency range into a plurality of time or time/frequency slots each ofwhich is associated with one of the ranges of priorities.

[0093] Each of the participants who is not immediately eliminated fromfurther participation owing to gross unsuitability (e.g., they arealready occupied or is already responding to a call) responds to thecall message by transmitting an indication signal in the appropriatetime or time/frequency slot in accordance with his respective priority.The indication time slots all start at a time relative to a time basecommon to all of the participants. It is important to note that for thisembodiment of the invention all those responding participants having apriority within the same range respond at the same time and frequency.As a result, substantially simultaneous indication signals are receivedby the control center from those participants having the highestpriority as determined at the current priority resolution according tothe protocol. The indication signals, which are preferably pulsed CW(i.e., they are pulsed signals at a particular frequency having noinformation content other than that given by the time at which thetransmission occurs and the frequency of the transmission), aresufficiently non-destructive with respect to other simultaneouslytransmitted indication signals and have at least sufficient pulse widthso as to permit an indication that at least one of the taxis hasresponded in a given time slot. In certain cases it may be necessary toadd some dithering or other variations to the signals so as to avoiddestructive interference between the signals.

[0094] Since the indication signals may, and typically do, overlap, evena fairly narrow bandwidth broadcast channel may be employed, there beingno requirement to discriminate between different indication signals inthe same priority slot. Furthermore, intermodulation effects between theindication signals in a given slot are not important, since only thepresence of at least one indication is required, and the intermodulationdoes not affect this determination.

[0095] The control center monitors any response and determines whichslots have a true indicator signal (as opposed to noise or othertransients). Preferably, the time slots are arranged in descending orderof priorities, such that the control center may ignore all slots afterthe first one (or some other small number of) “occupied” slot.

[0096] The control center targets all those responding taxis having thehighest priority range in respect of which a valid indication signal hasbeen received. Except as will be described below, taxis having a lowerpriority are excluded from further consideration.

[0097] If a predetermined criteria for stopping the targeting phase hasnot been reached, then an additional call is broadcast requesting allthose taxis which have a priority within the highest priority range torespond. The response of the taxis is similar to that sent in theprevious step except that the time or time/frequency slots now representsub-ranges within the highest indicated priority range or ranges. Ingeneral the call will include this range and may include an indicationof the protocol for dividing the slots among the priorities.

[0098] It should be understood that, for the more general case ofmultiple criteria, the priority vector may be a function of theiteration number and/or the priority range. Thus for example, the firstiteration may be used to eliminate taxis which are far away from thedestination without giving much weight to the idle (waiting) time. Thesecond iteration may give a greater weight to the idle time or to otherfactors. In general, taxis which have moved closer to the destinationsince the last call and have an increased priority may participate inthe second iteration even if they did not have the highest priority inthe previous iteration or were not detected as having this priority.Furthermore, a special slot (hereinafter also referred to as a “miss”trap) may be provided for taxis whose priority is now higher than thehighest range detected in the previous iteration. These taxis would takeprecedence over the other taxis by using the special slot.

[0099] This iterative reduction of the number of participants continuesuntil a predetermined criteria is reached. This criteria may includeconsideration of the priority resolution achieved. The criteria mayinclude a statistical estimate of the number of vehicles which have notbeen eliminated. For example, if in a given iteration in which asubstantial number of sub-slots have been allocated, only one or a fewsub-slots contain a response, it is then fairly certain that the numberof taxis left in the system is small (or even one) and the iterationprocess (and the targeting phase) is ended.

[0100] Another iterative approach which may sometimes be used is torestrict the responders in the first phase to a single range or alimited number of ranges. Assuming that the range of interest is between0-10 km from the customer. A first call would only ask for responsesfrom those taxis which are closer than 5 km from the customer. Thisdistance could be divided into ranges or a single range could be used.If there were no responses, then the call would request responses fromthose taxis in the range of 5-10 km. If there were a response, thenfurther delineation of the range would successively narrow the range ofdistances. In this method of restriction all positive responses to thequery are preferably broadcast in the same time/frequency slot.

[0101] Preferably, in the targeting phase no participants are actuallyidentified, and therefore it is not yet possible for the control centerto dispatch a particular taxi to the customer. Before this can be done,it is first necessary to complete a second, identification, phasewherein one of the taxis targeted at the end of the first phase isuniquely identified.

[0102] An additional call is broadcast or otherwise transmitted to theparticipants indicating that an identification phase is to begin. All ofthe targeted participants remaining at the end of the first phase areinvited to broadcast or otherwise transmit their identification codes inone of a number of identification slots (which may be time ortime/frequency slots, DS-CDMA or FDMA slots). These slots have aduration (or information bandwidth) commensurate with the information tobe transmitted by the taxis. The number of identification time slots isdetermined in accordance with the protocol and is application-dependent,and may be based on the number of participants which are believed to be(or estimated to be) left.

[0103] For example, in a dispatching system, the priority scale mayextend from a distance of 10 km from the customer location and theinitial priority resolution (so far as the distance criterion isconcerned) may be 1 km which is reduced during two successive iterationsto 100 m and finally to 1 m. At such a fine priority resolution it isnot to be expected that more than a small number of taxis will betargeted so a fast converging identification phase having only a fewtime slots ought to be sufficient for identifying one of the targetedparticipants. It is not suggested that a 1 m distance is significant indetermining priorities for taxis, however use of such fine distinctionsaids in reducing the number of taxis which participate in theidentification phase. However, as will be explained below, the protocolhas built therein sufficient discrimination to allow for possible errorsin the number of identification time slots allocated and to compensatefor such errors as required.

[0104] The identification slots are preferably not assigned in any way,and the taxis choose their slots in some random way. It can be expectedthat at for least some of the slots more than one taxi will broadcastits identification information. Such broadcasts probably can not be readby the control center which will choose the first taxi which it canidentify. If multiple dispatches are required to the same destination,as for example where there are too many passengers for one taxi, thesecond phase may have to be repeated several times until the requirednumber of taxis are dispatched. Furthermore, in extreme cases, it may benecessary to call for identification from taxis having a lower priority.

[0105] As in the targeting phase, a slot may be provided in theidentification phase for taxis having a higher priority than the call.These taxis may have moved closer to the destination or their signal maynot have been received by the receiver due to interference or blockage.

[0106] Alternatively, the stations may be identified in an alternativeembodiment of the identification phase in which a slot is assigned toeach of the remote stations (including those which are not targeted,since the system has no indication of those which are left). All of thestations which are targeted are invited to broadcast in theiridentification slot. One or more of these responding stations is thenchosen. Using this system of identification of the remote stations freesthem of the need to transmit any information bearing signal, simplifyingthe system.

[0107] Having described the overall method for iteratively targeting,during a first phase, successively fewer participants and then, during asecond phase, identifying a desired number of the targeted participants,there will now be described a specific application thereof to thescenario depicted in FIG. 1 and with reference to FIGS. 3 to 7 of thedrawings.

[0108] Referring then to FIG. 3, a customer 25 has requested a taxi.Shown within a circular target area 40 centered about customer 25 andhaving a boundary 35 at different distances from customer 25 are fourtaxi vehicles designated as V_(a), V_(b), V_(c), and V_(d). Vehiclesoutside boundary 35 are excluded from consideration.

[0109] Target area 40 is split into a plurality of concentric sectors ofwhich only the outermost sectors 42, 43, 44 and 45 are shown each havinga width ΔR and being radially disposed with respect to the customer.Adjacent sectors 42 and 43 or 43 and 44 or 44 and 45 are contiguousalthough for the sake of clarity and explanation they are shown in FIG.3 as separated from each other.

[0110] It will be noted that vehicles V_(b) and V_(c) are within thefirst (innermost) sector 42, vehicle V_(d) is within the middle sector44 and vehicle V_(a) is in the last (outermost) sector 45. Since it isdesired to allocate the task of servicing the customer to the vehiclewhich is closest to him, it is clear that one of the two vehicles V_(b)and V_(c) in the innermost sector 42 must be identified as the mostsuitable for the task. It will also be apparent that the number ofvehicles which exist in any particular sector is a function of the widthof the sector. Thus, if the width of each sector is increased from ΔR to3ΔR, it is apparent that vehicles V_(b), V_(c) and V_(d) will now existin the new, innermost sector comprising original sectors 42-44. In thismanner, the width of each sector ΔR constitutes a priority resolutionwith which a priority is assigned to the participating vehicles. Thecoarser (i.e. lower) the resolution, the more vehicles will answer theselection criteria and be rated at a particular priority associatedtherewith while the finer (i.e. higher) the resolution, the smaller thenumber of vehicles which answer the selection criteria and are ratedwith the corresponding priority.

[0111] Thus, after a first step of targeting in which a small group oftaxis is chosen, in a second step of targeting the highest prioritytaxi, a coarse resolution (finer however than that in the first step) isset as shown in FIG. 3 and a call message is transmitted by controlcenter 12 to all of the participants. The call message also preferablydefines a time interval ΔT which is divided into an equal number of timeslots Δt generally of equal width such that the total number of timeslots is equal to the total number of priorities: i.e. the number ofsectors. In a further preferred embodiment of the invention, frequencydiversity can be used to define multiple slots at the same time, each ofthe slots being at a different frequency distinguishable by the controlcenter.

[0112] Upon receiving the call message, each of the participating taxisdetermines its priority in accordance with the selection criteria,which, in the simplest case, is assumed to be solely the distance of theparticipant from the customer and within the maximum radius R_(max).Thus, vehicles V_(b) and V_(c) are both assigned the highest priority,while vehicles V_(d) and V_(a) n that order) are assigned successivelylower priorities. It should be noted that typically there may behundreds of vehicles in the target area 40; only a few are shown in thefigure for the sake of clarity. Further, each vehicle may have a handset(see FIG. 9) having a disabling switch by means of which the driver canprevent the transmission of a response message upon receiving an callmessage from control center 12. By such means he can go off duty, etc.

[0113] The active participants V_(a) to V_(d) now transmit an indicationsignal within the time slot Δt corresponding to their priority. Thus,vehicles V_(b) and V_(c) transmit an indication signal first; vehicleV_(d) transmits his indication signal second; and vehicle V_(a)transmits his indication signal third. In an actual situation, ofcourse, there may exist many time slots corresponding to a large numberof coarse resolution priorities and perhaps hundreds of vehicles willtransmit an indication signal in the same time slot. This, in itself, isnot important because all that matters during this first phase of theprocess is to determine the first time slot (or more generally, the slotrepresenting the highest priority) in which a vehicle transmits anindication signal.

[0114] This having been done, it is immediately clear which is thenearest sector to the customer in which at least one vehicle is locatedand therefore all of the vehicles in all of the other sectors may now beeliminated. In a practical implementation of such a system, thebroadcast and receive time for transmitting the call message from thecontrol center to the participants and receiving the first indicationsignal therefrom takes a short time. Thus, in a relatively short timeinterval thousands of participants in the field can be reduced to asmall number of potentially suitable participants for the task, withoutthe use of excessive frequency spectrum.

[0115] Furthermore, if a full duplex communication system is used, thecontrol center need not wait for the entire time ΔT, and can go on tothe next iteration or the next phase immediately when a first indicationsignal is received.

[0116] As explained above, this process is repeated iteratively as oftenas required, each iteration having successively finer priorityresolutions (i.e. sectors of successively decreasing width ΔR), until apredetermined resolution is reached. At this point, the width of theremaining sector is sufficiently small that only a small number ofparticipants are likely to be found therein. It is, of course, not knownhow many participants there are in this remaining sector sinceregardless of whether only one participant or many send an indicationsignal in a particular time/frequency slot, the control center does notreceive a message which is capable of uniquely identifying any one ofthose participants.

[0117] It should be noted that the receive time taken for the controlcenter to process a response from the highest priority participants is afunction of the number of time slots Δt. Thus, as the resolution isincreased, there will be more time slots and, since each requires aminimum transmit time, it might take longer to identify the highestpriority time slot. There is therefore a tradeoff between, on the onehand, increasing the resolution so as to identify the most suitableparticipant in fewer iterations and, on the other hand, increasing thecycle time of a given iteration by doing so. The choice of initialresolution and rate of increasing the resolution may be made based onthe number of participating vehicles and or a priori expectations of theresponses. Thus, the range of values of a priority which are assigned agiven sector may be based on the number of expected units having thatpriority. If distance is the sole criteria, the range of distance valuesmay be proportional to the distance so that the area of the sectorsassigned to each priority may be the same.

[0118]FIG. 4 shows a subsequent iteration of the targeting phase whereinthe priority resolution is increased and a further call signal istransmitted to the participants. The currently targeted participantsV_(b) or V_(c) in what is currently the highest priority sector 42 areassigned new slots according to the finer priority resolution and againtransmit indication signals during time slots corresponding to theirpriorities. As a result, it transpires that V_(c) has a higher prioritythan V_(b) and its indication signal is therefore transmitted first (orin a slot corresponding to a high priority, even if such slot is notfirst). However, from the perspective of the control center, there is noway of knowing how many participants exist in what is now the highestpriority sector. All that can be known is that at least one participanthas the priority.

[0119] Thus, while there may still be hundreds of participants in thetargeted sector, it is expected that with the increased priorityresolution only a small number of participants will now be targeted. Oneof these is now to be identified to respond to the request for service.During this second phase of identification, the control center assigns anew time interval ΔT-ID and divides this time interval into a number ofequal width time slots Δt (or time/frequency slots) related to theexpected number of participants in the highest priority sector 42. Theexpected number of participants in sector 42 is determined statisticallyas a function of the resolution of the sector ΔR and according to theapplication or according to the method described below. The onlyremaining targeted participant V_(c) in the highest priority sector 42now selects randomly one of the time slots and transmits, within therandomly selected time slot, an identification message whereby thesending participant can be uniquely identified.

[0120] In the more general case, where there are still a number oftargeted participants, the control center receives a plurality ofidentification messages some of which may, of course, have beentransmitted during the same randomly selected time (or time/frequency)slot. It is understood that where two signals are transmitted at thesame time and frequency, no information on the identity of thetransmitting participant is obtained in the absence of a capture effect.However, it is expected that at least one of the identification messagescan be uniquely identified and, in this case, the task is allocated to aparticipant which can be identified. Where possible, of course, in theinterest of speed, the task is allocated to the first uniquelyidentifiable participant.

[0121] If it is not possible to identify one of the participantsuniquely, the communication protocol allows for appropriate actionaccording to each particular situation. Thus, it may be that during thefinal iteration in phase one, no participants were targeted. This itselfcould be due to several different reasons: for example, the call messagemay never have reached the participants or, more likely, the response ofthe highest priority participants may not have been received, possiblyhaving been obstructed by an obstacle in its path.

[0122] Alternatively, possibly too many participants were targeted inthe final iteration of phase one and an insufficient number ofidentification time slots were allocated during phase two. In this case,identification messages may collide during all of the identificationtime slots, rendering it impossible to identify any one participant. Inthe more general case where more than one participant is to beidentified, it may also occur that too few identification messagesarrive in phase two owing to an insufficient number of participantshaving been targeted in phase one.

[0123] The various strategies for dealing with each of thesepossibilities from the point of view of the control center will now bedescribed with reference to FIGS. 5-6B which show state diagramsrelating to the targeting and identification phases, respectively. Inboth of these diagrams the following terminology is employed: PHASE-1.x:x^(th) iteration of phase 1; PHASE-2.x :x^(th) iteration of phase 2; IB:Control center's Broadcast Message; RD :Responders' signal Detectionand signal processing; IBPH1.x :Control Center's x^(th) Broadcastmessages in Phase 1; IBPH2.x :control center's x^(th) Broadcast messagesin Phase 2; ΔTRTPH1.x :x^(th) time interval for Responder's Transmissionactivity in Phase 1; ΔTRTPH2.x :x^(th) time interval for Responder'sTransmission activity in Phase 2; x :Number of iterations in Phase 1 or2 (application-dependent); NIP :Total number of iterations performed incurrent Phase; Limit1, Limit2 :Application-dependent maximum number ofiterations for Phases 1 and 2, respectively. n :predetermined number ofsuccessful iterations PS :Priority slot

[0124] Thus, referring to FIGS. 5A and 5B, if during a successiveiteration, no indication signal is received (i.e. a MISS is detected),the initiator requests at least once that all of the participants whohave not yet been identified transmit a respective indication signal andthis is repeated until an indication signal is received or for a maximumnumber of iterations determined in accordance with the protocol.Thereafter, whilst the resolution is higher than a minimum resolutiondetermined in accordance with the protocol (and the iteration processhas not been terminated for some other reason), further prioritieshaving a coarser resolution are assigned to all of the participants whohave not as yet been identified, or until the resolution reaches theminimum resolution.

[0125] Another way of checking if a MISS is true is to provide anadditional slot during which all of those stations which should havebroadcast during the designated priority slots will broadcast again. Ifno signal is received during this slot, the MISS is verified.

[0126] Referring to FIGS. 6A and 6B, if during any iteration noidentification message is received by the control center and duringpreceding iterations fewer than the desired number of identificationmessages were received so as to permit identification of the respectiveparticipants or if invalid data were received, there is further includedthe step of the control center requesting at least once that anycurrently targeted participants who have not yet been identifiedre-transmit their identification message.

[0127] If fewer than the desired number of valid identification messagesare received so as to permit identification of the respectiveparticipants owing to the occurrence of more than one identificationmessage arriving in the same identification slot or to any other reasonsuch as receipt of erroneous data, thereby rendering it impossible todetermine the respective identifications, the following courses ofaction may be taken.

[0128] One possibility is for the control center to allocate to all ofthe targeted participants still remaining and who have not yet beenidentified a greater number of discrete identification time slots thanthe number previously allocated, and to invite the targeted participantswho have not yet been identified to transmit a respective identificationmessage during one of the new identification time slots. In other words,the number of targeted participants is maintained but moreidentification time slots are allocated so as to increase theprobability that the desired number of valid identification messageswill be received by reducing the probability of collisions.Alternatively, the taxis can be required to choose a random number whichcan then be compared to some reference number to eliminate some of thetaxis or which can be used in changing the priorities of the taxis toeliminate some of them. Alternatively, an additional criteria may beadded to reduce the number of participants.

[0129] Alternatively, if the protocol allows a maximum priorityresolution, then so long as the current priority resolution is lowerthan the maximum priority resolution, phase one can be repeated asrequired for a maximum number of iterations determined in accordancewith the protocol at successively finer priority resolutions, until themaximum resolution is reached in respect of all of the participants whohave not as yet been identified. This causes fewer participants to betargeted and again reduces the probability of collisions in phase twowhen any newly targeted participants are identified.

[0130] If, on the other hand, during a successive iteration, fewer thanthe desired number of valid identification messages are received so asto permit identification of the respective participants owing to aninsufficient number of participants having been targeted duringpreceding iterations, then the opposite must be done. Thus, so long asthe resolution is higher than a minimum resolution determined inaccordance with the protocol, phase one is repeated as required for amaximum number of iterations determined in accordance with the protocolat successively coarser priority resolutions until an indication signalis detected or the minimum resolution is reached. This process isperformed in respect of all of the participants who have not as yet beenidentified and, by targeting participants in phase one who were notpreviously targeted, increases the probability that the desired numberof newly targeted participants will subsequently be identified in phasetwo.

[0131] If, during phase one, no indication signal is received inresponse to a call message, the control center requests at least oncethat the participants re-transmit an indication signal. On receipt ofthe call message, the participants assign themselves priorities andtransmit respective indication signals during a corresponding indicationtime slot. This covers the possibility that the call message neverreached the targeted participants or, alternatively, that theirresponses never reached the control center.

[0132] In all of the above cases, data is stored in respect of anyparticipants who have already been identified and subsequent iterationsare performed only to identify additional participants.

[0133] The protocol includes at least one termination condition wherebyfurther iterations are not performed even if no indication signal hasbeen received and/or if fewer than the desired number of participantshave been identified. This is necessary to avoid an infinite loop beingexecuted in the event that, in a particular application, there are notenough participants who can be identified.

[0134]FIGS. 7A and 7B show a timing diagram relating to the flow ofinformation between the control center and the participants V_(a),V_(b), V_(c) and V_(d) during the example of FIGS. 3 and 4.

[0135] It will be noted that in the initial phase of targeting, eachparticipant selects a time slot according to his respective priority,such that participants with the highest priority transmit first.Consequently, as soon as the control center receives a response from theparticipants, the highest priority may be determined immediately inaccordance with which time slot data was first received. Furtheriterations may now be effected, as required, there being no need even toawait the responses of lower priority participants. This results in veryrapid convergence of the targeting phase to the priority rangecontaining the most suitable participant. This requires a full duplexsystem. FIGS. 7A and 7B show the timing diagram for a half duplexsystem.

[0136] In another embodiment of the invention priorities may be assignedaccording to a measured elapsed time since participants have performedsome activity. For example, priorities are assigned to taxis accordingto the time they have been idle.

[0137] In a first iteration of phase one, the mutually common priorityscale relates to an elapsed time of say 3 hours and the priorityresolution is say one-half hour. Thus, each interval in the priorityscale corresponds to an elapsed time of one-half hour.

[0138] In a second iteration of phase one, the mutually common priorityscale relates to an elapsed time of one-half hour and the priorityresolution is 2.5 minutes. Thus, each interval in the priority scalecorresponds to an elapsed time of 2.5 minutes. If during the firstiteration a signal was received in the time slot of two to two andone-half hours, then the time slots in the second phase may have aresolution of 2.5 minutes and span the range between these limits.

[0139] If this is considered to be sufficiently fine so that not toomany participants will have the same priority, the process is terminatedafter only two iterations. It is now appropriate to implement phase twowherein one of the targeted participants is identified.

[0140] It will be understood that since, during the identificationphase, a participant may be, in effect, selected randomly, it cannot beassured that the identified participant is actually the one who haswaited the longest. However, it can be said with certainty that theidentified participant has the highest priority to within the priorityresolution (in this case 2.5 minutes).

[0141] If, notwithstanding the above expectation, it becomes impossibleto identify a single participant in phase two owing to too manyparticipants having been targeted during phase one, then, as explainedabove, several options are available. More identification time slots canbe allocated in phase two or, alternatively, a further iteration inphase one can be performed at an even finer priority resolution, forexample 6 seconds, before repeating phase two in respect of a smallernumber of targeted participants or one of the other options describedabove may be employed.

[0142] In all of the embodiments described above, at least two phasesare required to identify a targeted participant. Thus, during a firstphase, participants are only targeted and are identified during asubsequent second phase. However, according to another preferredembodiment of the invention, provision may be made for identifying aparticipant during the first phase by transmitting an identificationmessage as the indication signal. The identification message can bedecoded in the particular circumstance that only one participant has thehighest priority, so that only one indication signal is transmitted inthe highest priority time slot, and there is a sufficiently long timeinterval between receipt of successive indication signals by the controlcenter to allow decoding of the identification signal before a lowerpriority indication signal arrives in a subsequent indication time slot.Alternatively, the identification time slots are made long enough sothat different slots have minimal or no overlap. In this particularcase, the second phase of identifying the targeted participants iseliminated. It should be noted that, generally, this embodiment of theinvention is less efficient than the embodiment which usesnon-information bearing signals in a first, targeting, phase to reducethe number of vehicles in a second, identification, phase.

[0143] Yet a further consideration relates to the possibility that thehighest priority participant may not be targeted in phase one owing to amalfunction. Thus, for example, his indication signal may not bereceived, having been obstructed by an obstacle in his path or hissignal is subject to fade. This may not matter if other participantshaving the same priority have nevertheless been able to transmitindication signals, since if the indication time slot having the highestpriority is determined and all the participants associated therewith aretargeted, even a participant whose indication signal was lost will stillbe targeted. However, if a sole participant's indication signal is lostthis could prevent correct determination of the highest priorityparticipant.

[0144] The protocol can take this possibility into account by reserving,preferably, the first indication time slot in the next iteration forexclusive transmission therein by a non-targeted participant having ahigher priority than that of the targeted participants. The controlcenter then transmits a call message inviting the targeted participantsto transmit a respective indication signal during any one of theindication slots except the reserved indication slot.

[0145] So far as newly targeted participants are concerned, the processis essentially unchanged; each of the newly targeted participantstransmits an indication signal during one of the unreserved indicationslots according to his respective priority. However, any previouslynon-targeted participant having a higher priority than that of the newlytargeted participants transmits a respective indication signal duringthe reserved slot. This slot is referred to herein as an“Inter-Iteration Miss/Trap Control Slot.”

[0146] In a further preferred embodiment of the present invention, atleast one iteration of the targeting phase includes a control slot,which is reserved for simultaneous transmission by all thepriority-bearing participants of the iterative stage responding to thecall sent from the control center. According to this preferredembodiment, each priority-bearing participant transmits twice, onceduring its characteristic-indicating slot as described above and onceduring the control time slot. This control slot is referred to herein asan “Intra-Iteration Miss/False Control Slot.” This control slot providesan independent indication of transmission by at least one participant inresponse to the control center call which initiated the iteration.

[0147] The additional indication obtained from the Intra-IterationMiss/False Control Slot will generally economize on both the totalprocessing time and the total transmission time of the targeting phase.For example, if no transmissions are received in this control slot, itmay be assumed that any transmissions received in thecharacteristic-indicating slots are due to a false alarm error and,thus, further processing and transmissions derived from the lastiteration are avoided. Similarly, in a “miss” error situation in whichno participants are targeted, a transmission received in this controlslot indicates a possible “miss” and the last iteration is preferablyrepeated. Thus, the use of a control time slot improves the reliabilityof the targeting phase.

[0148] It should be noted that for improved reliability of the detectionprocess at the control center receiver, the remote station transmittermay perform a more sophisticated transmission process that includesdiversity techniques based on randomly varying the amplitude and or thephase of the transmitted signal, so that any correlation betweentransmitters will be reduced when detecting a sequence of transmissionslots.

[0149] To further improve the reliability of the targeting phase, apreferred embodiment of the invention employs a majority votingtechniques in which an odd number of slots greater than one, for examplethree slots, are assigned to each range of characteristic values.According to this preferred embodiment, each participant of a givencharacteristic value transmits an indicating transmission during each ofthe time slots assigned to the range. The given characteristic value istaken into account only when indicating transmissions are received in amajority of the slots assigned to the value, for example by two slotsout of three. When indicating transmissions are received only by aminority of the slots assigned to the value, for example by one slot outof three, the response is preferably ignored. It should be appreciatedthat the odd number of slots assigned to the given range ofcharacteristic values may be distributed among slots that have minimumcorrelation.

[0150] It is appreciated, however, that such majority voting schemeswhich improve the over-all reliability of phase one, consume substantialtransmission and processing time. Therefore, such schemes are preferablyapplied selectively, in potentially problematic situations. For example,majority voting may be employed only after a predetermined number ofprevious false alarms and/or “misses” have been detected using thecontrol slot technique described above or otherwise.

[0151] Within a single iteration of phase one, the assignment ofpriorities to each participant may be performed in respect of adifferent sub-set of selection criteria for at least some of theparticipants. In effect, this permits different search strategies to beexecuted each in respect of a respective indication slot. For example,the first indication slot having the highest priority may relate to allparticipants who are located within a radius of 10 m from the customerwithout any further restriction; whilst the second indication slot mayrelate to all participants who are located within a radius of 25 m andwho have been awaiting instructions for more than 20 minutes. By thismeans Boolean OR search or other search strategies can be performed in asingle iteration.

[0152] Furthermore, during each iteration the participants mayoptionally assign themselves a priority having a magnitude outside thepriority scale so as not to be targeted by the initiator. This can bedone if, for example, a participant is otherwise occupied or for anyother reason does not wish to receive instructions.

[0153] During a particular iteration the priorities assigned to eachparticipant are generally absolute with respect to a mutually commonscale which itself is external to the participants and independentthereof. However, between successive iterations the priority scale maywell relate to different combinations of selection criteria. By thismeans finely tuned search strategies can be performed whereby allparticipants answering to a first combination of selection criteria aretargeted during a first iteration, whilst all of the targetedparticipants answering to a different combination of selection criteriaare targeted during a successive iteration.

[0154] Alternatively, the combination of criteria which make up thepriority may be information dependent and, for example, different groupsof slots may relate to different combinations of criteria.

[0155] Once a sufficiently small number of participants are targetedsuch that, in accordance with the protocol, identification of a desirednumber of participants is likely to yield a successful outcome, thesecond phase described above is commenced. The number of identificationslots to be allocated to the targeted participants is calculated byfirst estimating the number of targeted participants remaining at theend of phase one. The number of identification slots is then calculatedaccording to the estimated number of targeted participants who musttransmit respective identification messages, so as to reduce the totaltime required to identify the required number of participants.

[0156] In this connection, it will be realized that there exists atradeoff between allocating too many and too few identification timeslots. Specifically, allocating too many identification time slotsreduces the probability of a targeted participant selecting an earlyidentification slot, thereby increasing the time required to identifythe highest priority participants. On the other hand, allocating too fewidentification time slots increases the probability that more than oneparticipant will select the same identification time slot. In this case,the resulting collision of more than one identification message makes itimpossible to identify the respective participants, requiring furtheriterations and again increasing the identification time. In practice thenumber of identification time slots may be minimized by increasing themaximum priority resolution in phase one in order to target no more thanthe expected number of participants who are to be identified in phasetwo, or by using a random process to eliminate some of the participants,such as that described above.

[0157] In the specific embodiments described above the process ofassigning priorities to each of the participants is performed within theparticipating vehicles themselves since only they know their locationsrelative to the customer. Moreover, the onus of tracking theparticipants' movements in terms of their location, availability,occupancy, loading and all the other selection criteria which may be ofsignificance is now passed to the participating vehicles themselves asopposed to most hitherto proposed systems wherein a central dispatcherhad to keep track of all these parameters.

[0158] As a result of the above, the communication channel between thecontrol center and the participants may be of relatively narrow spectrumwidth compared with that of hitherto proposed systems. Additionally, thetask of targeting potentially suitable participants is distributedamongst the participants themselves rather than being determined solelyby the control center. Such distribution results in a reduction ofcomputing power being required by the control center.

[0159] While the selection criteria must obviously be known to theparticipating vehicles, the manner in which this is made known can bevaried according to circumstances. Thus, for example, the selectioncriteria may be fixed and known in advance to the participants (in whichcase the selection criteria are not subject to change). Alternatively,the selection criteria may be determined on-line by the control centerand then transmitted to all of the participants together with the callmessage.

[0160] Thus, in the particular example described above, during the firstphase of targeting it may be predetermined that each sector has a widthof 10 km and that in subsequent phases, the width of each remainingsector is reduced, for example, by a factor of 10 until the sector has awidth of only 10 m, whereupon all those vehicles within the 10 m widthsector send an identification message; or, alternatively, the width ofeach sector in each respective phase of allocation may be transmitted tothe participants by the control center. In order to reduce the number ofparticipants in the identification phase, the resolution in thetargeting phase may be increased artificially, i.e., past the point atwhich it is meaningful.

[0161] It should also be noted that once a particular participant hasbeen uniquely identified to perform a task, he is notified of this inthe normal way by the control center, in any one of a number of wayswhich are well known in the art as for example, by voice over acommunication channel or by text or other data transmission.

[0162] Furthermore, although in the preferred embodiment describedabove, one participant is uniquely identified as the most suitable, infact it may sometimes be appropriate to omit the second phase ofidentification altogether. In such cases, the participants having thehighest priority are not uniquely identified as individuals but all areidentified as a group. One such situation relates to an improvement ofservice in particular areas. In this situation the number of taxis in agiven area is monitored and additional cars are sent in to the area ifthere are not enough cars in the area. The number of cars may beestimated statistically, for example, from the number of slots havingresponses.

[0163] In a further preferred embodiment of the present invention, thenumber of participants complying with given criteria is estimated basedon a down-sampling technique in which only a portion of the complyingparticipants actually respond to a call from the control center. Forexample, the participants may be assigned a given response probabilitysuch that only a given percentage, for example ten percent, of thecomplying participants respond to the call. Response time-slots arepreferably randomly selected by the different participants. If thenumber of response slots is substantially greater than the number ofresponses, i.e. the number of complying vehicles times the responseprobability, the number of detected responses generally corresponds tothe number of responses which, in turn, is a down-sampled indication ofthe number of complying participants.

[0164] Down-sampling is particularly useful for situations in which thenumber of expected responses is high, such as for estimating the numberof vehicles in a given area, when very course selection criteria areapplied. On the one hand, in such a case the number of responses islarge enough to be statistically reliable, provided that the number ofresponders is small enough compared to the number of slots dedicated forthis purpose. On the other hand, the smaller number of participantstransmitting simultaneously reduces the total transmission power and,thus, prevents occasional bursts of powerful transmission which may bein violation of FCC co-channel interference or other regulations.

[0165] Another application of the system of priority assignmentaccording to the invention is in the assignment of available lines forcar-phones or transceivers. Presently, available lines are allocated ona when available basis. Thus, one unlucky user may wait a long timewhile a lucky user may get an immediate line. In a preferred embodimentof the invention, when a user wants a line, he indicates this either bypressing a call button or by lifting his receiver. A computer chipassociated with the car-phone notes the time at which a line wasrequested.

[0166] Lines (communication channels) are allocated on a waiting timebasis. In operation, a control center broadcasts a call for prioritiesin accordance with a targeting phase of the present invention. Thepriority is assigned according to waiting time, and the individualphones broadcast signals during time slots assigned according to theirwaiting time or by some other special priority. During a secondidentification phase one of the phones is identified, in the same manneras described above, and is given the available line.

[0167] Referring now to FIG. 8 there is shown schematically theprincipal features associated with the control center shown in FIG. 1.Thus, there is provided a transceiver and modem 50 coupled to an antenna51 for effecting bi-directional communication with the participants(taxis) and being connected to a message processor 53 which is coupledto a computer 54. Message processor 53 receives non-demodulated signalsfrom the transceiver and determines which slots contain signals for thetargeting phase and identifies the participant(s) in the identificationphase. A preferred embodiment of such a receiver is shown in FIG. 15.

[0168] A service request is effected by customer 25 by telephoning hisnearest taxi rank and then dialing his telephone number, the requestbeing routed to the computer 54 via a Public Switched Telephone Network(PSTN). The computer 54 converts the customer's telephone number to acorresponding location based on a data base stored in the computer.Alternatively, such communication can be effected via an operator. Aterminal 56 is coupled to the computer 54 for allowing an operator toenter commands and display data. In addition the system also allows forvoice signaling to a dispatcher at the control station or for voicecommunication between the taxi driver to the customer.

[0169]FIG. 9 shows the principal components associated with aparticipant allocation unit 60 located in each of the vehicles.Allocation unit 60 preferably includes a transceiver 61 coupled to anantenna 62 for effecting bi-directional communication with thetransceiver 50 in the control center 37. Transceiver 61 is connected toa vehicle computer 64 coupled to a microphone/handset 65 providing ahuman interface between the vehicle computer 64 and the correspondingtaxi driver.

[0170] A Global Positioning System 66 (GPS) or other positiondetermining system as known in the art receives positioning data via anantenna 68. The Global Positioning System 66 is coupled to the vehiclecomputer 64 and functions as a positioning means for providingpositioning information relative to a predetermined origin in respect ofthe corresponding participant. Thus, once the location of the customeris provided to the vehicle computer 64, the latter, being coupled to theGlobal Positioning System 66 is able to determine the relative locationof the participant to the customer and thus determine the participant'spriority.

[0171] Associated with the vehicle computer 64 is a storage means 70 forstoring the protocol according to which priorities are assigned. Alsostored in the storage means 70 are any singular areas which can affectthe actual route e.g. obstructions such as rivers, road blocks and so onwhich result in the actual route distance being longer than it wouldotherwise be. As explained above, the handset 65 allows the driver toassign himself a priority outside the range of the priority scale and,by such means, to exclude himself from the process of targeting. It alsoincludes a microphone for establishing voice contact with the controlcenter, as well as paging means for obtaining a text message therefrom.

[0172] The system described above may include a full duplex broadcastnetwork such that the control center does not need to await responsesfrom all of the participants before targeting the highest priorityparticipants. Thus, specifically, as soon as a valid response isreceived by the control center, the participants corresponding to theresponse can immediately be targeted or identified whilst informingother participants to stop transmitting indication signals oridentification messages. This permits the steps of targeting and/oridentifying participants to be effected more quickly. However, theinvention may also be employed in a simplex (i.e. half-duplex) broadcastnetwork, albeit at the expense of longer targeting and identificationtimes since the control center cannot transmit to the participants untilall their responses have first been received and validated.

[0173] It will be appreciated that, instead of employing a GlobalPositioning System, other systems for determining a participant'slocation can equally well be employed. For example, a route schedulerbased on dead reckoning responsive to each participant's location can beused for determining a route having minimum distance. Such a routescheduler might possibly comprise sensors located at intervals along theroad for sensing a passing vehicle's presence and for transmitting tothe vehicle data representative of its location relative to a specifiedlocation for error correction. Typically, such a route scheduler has amemory for storing therein a scaled contour map so that an optimal routecan be determined taking into consideration the nature of the terrain.Likewise, prevailing traffic conditions can be fed into the routescheduler at regular intervals of time, so that traffic jams, roadworkand so on can be considered when determining the optimal route.

[0174] In the foregoing description it has also been assumed that asingle channel broadcast network is employed. However, this is by nomeans essential and a centralized controlled trunking system having atleast two channels may equally well be employed. This permits more thanone task to be handled simultaneously each on a different broadcastchannel. Thus, in the case of a two channel broadcast network, forexample, having first and second channels, each call message istransmitted via a broadcast control channel so as to be received by allthe participants associated with the first channel. Upon determiningthat he has not been targeted by the control center, a participantstarts to measure elapsed time and waits a predetermined elapsed timelocked on to the first channel and thereafter returns to the broadcastcontrol channel for receiving further call messages.

[0175] The period of time during which a non-targeted participantremains locked on to the first channel is of sufficient duration toallow an updated priority to be assigned to the participant. Owing tothe dynamic variation in a participant's status, it may occur that, withan updated priority, a previously non-targeted participant becomestargeted in the next iteration. Thus, the period of time during which anon-targeted participant remains locked on to the first channel mustfurther be of sufficient duration to allow a corresponding indicationsignal and/or identification message to be transmitted by theparticipant to the control center, whereby the control center may targetand/or identify the participant.

[0176] Alternatively, the call message may be transmitted via abroadcast control channel so as to be received by all the participantsassociated with the first channel and, upon determining that he has notbeen targeted by the control center, a participant receives from thecontrol center an instruction to return immediately to the broadcastcontrol channel. This immediately frees a non-targeted participant toparticipate in a subsequent search strategy on the second channelrelating to a different task.

[0177] According to yet another variation, an initial call message istransmitted together with the selection criteria via a broadcast controlchannel so as to be received by all the participants associated with thefirst channel. Each of the participants receiving the call messageassigns to himself from the priority scale a respective priorityrepresentative of his relative suitability in accordance with theselection criteria and transmits an indication signal during arespective indication slot. Only the targeted participants remainswitched to the first channel and subsequent call messages aretransmitted only to those participants who have been previously targetedby the control center. This again frees a non-targeted participant toparticipate in a subsequent search strategy on the second channelrelating to a different task.

[0178] It will be appreciated that whilst the invention has beendescribed with particular application to a taxi dispatching service, theinvention has more general application wherever one or a group amongst aplurality of participants is to be targeted in accordance with theirrespective suitabilities based on at least one selection criterion. Itwill further be understood that, whilst the preferred embodiment hasbeen described for the sake of simplicity with regard to only twoselection criterion (i.e. distance and waiting time), in practice alarge number of selection criteria may be employed, all having differentrelative weights, whereby an integrated search strategy may beimplemented.

[0179] It will also be understood that whilst the invention has beendescribed with particular reference to 2 dimensional terrain, it canequally well be applied in 3 dimensional space and is thus suitable forair or space travel, as well as land and sea.

[0180] Mention should also be made of the variable parameters inassociation with which the protocol functions. These are generallyapplication dependent and typically are provided with default valuesbuilt into the protocol. Thus, if distance is one of the selectioncriteria, this fact may be represented by a default value of anassociated parameter. Likewise, the lower and upper bounds of thepriority scale and the priority resolution associated with eachiteration in phase one can been assigned to respective parameters eachhaving corresponding default values.

[0181] Any unassigned parameters must, of course, have values assignedthereto prior to initiation of phase one. This can be done during theinitiation of the process prior to transmitting the first call messageto the participants. However, in certain applications, all theparameters may have pre-assigned default values which are acceptable forthe application. In this case, the call message merely starts theprocess enabling the participants to determine the appropriate priorityscale and assign themselves respective priorities at the appropriatepriority resolution; there being no need to inform any of theparticipants of the boundary values of the priority scale or of thepriority resolution or indeed of the selection criteria.

[0182] While the invention has been described with particular referenceto a wireless broadcast network, it will be appreciated that theinvention is capable of much more general application. For example,hard-wired communication systems may also employ the principles of theinvention in which case the indication signals need no longer be CW. Insuch cases the dynamic variables would generally not be position;however the system is generally applicable to systems with any set ofdynamic variables.

[0183] The principles of the invention may also be used in a routingsystem, for example to a system which identifies buses or other vehicleswhich are delayed and adjusts the speed and/or location of other busesto compensate therefor. In the first (targeting) stage of thisutilization of the invention, the priority would for example be based onthe amount of time that a vehicle is behind schedule. Vehicles which arebehind schedule more than a predetermined amount would then be targetedand identified in a second (identification) stage. Preferably, theidentified bus would then be asked for its exact position. Due to thefixed lineal nature of bus routes, the position of the bus on the routeis a one dimensional function, i.e., the distance along the path.

[0184] A query would then be sent to other busses on the same bus lineasking them for their positions and, optionally, where they stand inrelation to their schedule. Based on this information, a control centerwould determine corrective action to provide improved service, which mayinclude steps such as speeding up some buses, as for example byoperating them in a skip-stop fashion, slowing some buses down, keepingsome buses from leaving the terminal or adding new buses to the route,perhaps at some intermediate point on the route. Some indication ofoccupancy of the buses would help to avoid sending full or almost fullbusses to load additional passengers when less full buses are available.Such indication, which could, for example, be keyed in by the busdriver, would help in optimizing routing decisions. Suitableinstructions would, of course, be transmitted to these busses after acorrective action plan is formulated.

[0185] Alternatively, information on deviations from schedule areignored, and a revised schedule is based only on the position of thebuses and optionally on their occupancy.

[0186] The principles of the invention are also applicable to a routingsystem for determining slow areas of traffic and rerouting trafficaround such areas. In such a system a large number of participatingvehicles are queried as to the delays they are experiencing, and thedelay time is one example of a “characteristic value” for the firstphase of this embodiment. When a vehicle experiences a delay above athreshold, the position of the vehicle is determined in the secondphase. It should be noted that no identification signal per se istransmitted in the second phase, instead a position signal is broadcast.Preferably, the delay is also verified by the driver of the vehicle toavoid false alarms.

[0187] Once the position of the targeted delayed vehicle is determined,a new first (targeting) stage determines those vehicles that are closeto the specific delayed vehicle, and determines, by successive secondstages, the extent of the delay as a function of the time of the delay.Furthermore, by making multiple queries, the traffic conditions can beestimated. Based on this information, the seriousness of the delay maybe determined and corrective action, such as re-routing of othervehicles, may be started. In particular, information on the trafficconditions and the geographical extent of the delay may be transmittedto vehicles which have routing apparatus of types which are known in theart, to be used by these apparatus for determining the optimum route forthe receiving vehicle.

[0188] In an alternative preferred embodiment of the invention, thefirst query requests responses only from vehicles which are experiencingdelays greater than a given time (or which are moving at an averagevelocity of less than a given velocity). Those vehicles which meet thecriteria then broadcast a signal in a time or time/frequency orfrequency slot which is indicative of the absolute position of thevehicle. As in some of the previous embodiments of the invention, it isexpected that more than one vehicle will broadcast in a particular slotand the system is interested, at least at this stage, only indetermining if there are vehicles which are experiencing delays of agiven magnitude.

[0189]FIG. 10 shows an initial map generated by such a method, whereinthe area represented by a pixel (slot) may, for example, be of the orderof 250 to 1000 meters square.

[0190] In a preferred embodiment of the invention, the system thendetermines, based, inter alia, on the extent of the various contiguousareas which shows positive responses, a smaller area or areas forfurther study. Preferably, the system then broadcasts a further queryrequesting those vehicles within the more restricted area which have atleast a given delay (which may be the same as or different from thatused in the first query) to broadcast in a position slot using a finerresolution, for example, 100 to 250 meters. Based on the responses tothis query a second map such as that shown in FIG. 11 is generated. Ascan be seen from FIG. 11, various branches of a road network radiatingfrom an intersection, designated as A-F in FIG. 11, can be identified.To improve the usefulness of the display, a background map, such as aroad map may be displayed underlying the displays of any of FIGS. 10, 11or 13.

[0191] In the event that additional information relating to the delay isdesired, further queries can be made. For example, vehicles which aretraveling toward the intersection can be requested to broadcast in aslot which corresponds to the slot they are in and to their velocitytoward the intersection. This allows for generation of the graph shownin the lower portion of FIG. 12. Additional slots may be used for thegeneration of other information regarding the responding stations. Suchinformation may also be graphed as shown in the upper portion of FIG.12.

[0192] Alternatively or additionally, a map which shows the averagevelocity of the vehicles toward the intersection as a function of theposition can be generated. Such a map is shown in FIG. 13. To acquirethe information needed for generating such a map, a number of queriesmay be made, each requesting an indication from all vehicles within thearea of interest having a given average velocity toward theintersection. The responding vehicles would broadcast their indicationsignals in slots corresponding to their position. In the map of FIG. 13the velocity for a given pixel is determined, for example, as theaverage velocity of the reporting slots for that position. In a displayof the map of FIG. 13, the velocity toward the intersection can, forexample, be displayed as a gray scale value or as a color, with forexample red being the highest delay and blue being a minimum displayeddelay.

[0193]FIG. 14, which is a generalized block diagram for a system usefulfor performing the IVHS function described above, shows a base stationor control center 91 having a control center transmitter 79 whichbroadcasts queries and optionally other signals to vehicles on commandfrom a control computer 80. A remote vehicle 85 (only one vehicle isshown for simplicity) receives the query at a vehicle receiver 84 andtransmits commands to a microprocessor 86, based on the queries itreceives from the control center.

[0194] Microprocessor 86 also receives information regarding the statusof the vehicle from one or more information generators and sensorsindicated by reference numeral 88. This information may be sent by thesensors on a regular basis or may be sent on command from themicroprocessor.

[0195] Microprocessor 86 is then operative to command vehicletransmitter 90 to transmit indication signals (or if requiredinformation bearing signals) in a suitable slot in accordance with theinformation received by microprocessor 86.

[0196] The indication (or other) signals are received by a controlcenter receiver 92 and processed by receiver 92 and computer 80. Whilethe operation and construction of the apparatus designated by referencenumerals 82, 84, 86 and 90 is straightforward and needs no furtherexplanation, the operation of receiver 92 is usefully expanded upon withreference to FIG. 15.

[0197] The system described above is based on a central decision makerwhich receives information from vehicles, plans the routing for eachvehicle and then broadcasts a route or route changes to the individualvehicles. This type of system has the advantage that the routing foreach vehicle takes account of the routing for the other vehicles and thecontrol center in computing the routings can balance the routings tocause minimum delays. The disadvantage of such a system is the largebandwidth required to notify the individual vehicles of their individualcorrected routes.

[0198] A second approach for routing systems which has been suggested isto have each of the vehicles compute its own route, based on someinformation about the present status of traffic which it receives from acentral transmitter. While such systems require only a limitedbandwidth, the routes computed by the individual stations cannot takeinto account the future effects of the routes of other vehicles.

[0199] In a preferred embodiment of the invention, vehicles computetheir own routing and then report, in response to a query, theirexpected time of arrival at locations that are known to have a highincidence of traffic jams and slowdowns (and preferably also additionallocations which do in fact have such slowdowns). Preferably, suchreporting is performed using the slot method of transmission which doesnot identify the individual vehicles. Since a large number of vehiclesis involved, down-sampling as described below may be effectively used toestimate the numbers of vehicles which pass the locations.

[0200] Additionally or alternatively, the future development of existingslowdowns can be estimated from the prior development of the slowdowns,the rate of change of the length of the slowdown and the average speedof the vehicles which are within the slowdown. Such information can bemade available to the vehicles based on comparison of the development ofslowdowns which are detected by the methods which have been previouslydescribed above.

[0201] Based on the estimates of the numbers of and times of arrival ofthe vehicles at the trouble spots, information on future expectedtraffic jams is generated by the central station and broadcast to thevehicles which update and recalculate their individual routings. Thisrecalculation of routs, broadcast of times of arrival at trouble spotsand estimations of future traffic jams and slowdowns gives each vehiclethe information required to make a distributed system effective inavoiding future problems, without the huge bandwidth requirements ofcentral calculation of the routes for the vehicles.

[0202] Such a distributed method may be applied to fleet management andother systems.

[0203] For example, in a preferred embodiment of the invention, theredistribution of a a fleet of taxicabs from a, present, actualdistribution to a desired redistribution is accomplished by having thetaxicabs choose, based on a predetermined algorithm and informationtransmitted to them by a central station, which taxicabs will move to anew location. In this redistribution procedure, a present distribution(containing numbers of taxicabs in a region, without necessarily anyindication of the position of individual identifiable taxicabs) and anew desired distribution is transmitted to all of the taxicabs. Based ona predetermined protocol, each of the taxis will decide locally if theyare to move to a new location. Preferably, a decision by each taxi isbased on a statistical model whereby generally approximately the correctnumber of taxis decide, on their own, to move to new sites. Thisprotocol may consider parameters such as the location, idle time anddistance of individual taxi from the area which need additional taxis,as well as the present distribution of taxis and what, statistically,they will choose to do, based on the protocol.

[0204] Another similar application of the invention is bus fleetmanagement, where bus distribution information, occupancy information,connection times, location distributions, etc., is broadcast to the busfleet to enable the buses to make distributed decisions. In particular,based on the information received, a bus may, in effect, instruct itselfto skip a bus stop, wait (or not wait) for a connection with anotherbus, to leave a starting point early (or late), etc.

[0205] Generally speaking, the RF signals transmitted by the vehicle maybe at any frequency slot. It is to be expected (both for the IVHSapplication and for the dispatching application described above) thatthere will a certain amount of frequency diversity caused by theimperfect accuracy and stability of the vehicle transmitters 90. Theslots are wide enough to accommodate this diversity.

[0206] Furthermore, often the system utilizes very large numbers ofvehicles. If too many of these vehicles (in some particular situation)transmit in the same slot, then the total power transmitted may exceedauthorized ERP or dynamic range restrictions. To overcome this problemlonger, lower power, pulses may be used for indication signals.Furthermore, if a single receiver is used for receiving signals for allof the slots, intermodulation effects may cause spurious signals toappear in slots for which no actual signals have been received.

[0207] These problems as well as near-end to far-end transmissionproblems are substantially solved by the system shown in FIG. 15 and bycertain constraints placed on the system which are not shown in FIG. 15.

[0208] With respect to excess power problems, if it is expected thatmany vehicles may transmit in a particular slot, the queries can bedesigned so that fewer than the total number of vehicles will respond,whenever this is possible. This can be accomplished, for example byhaving the vehicles choose, statistically, which vehicles will respondwithin a given percentage of the total number of vehicles. The powertransmitted by the vehicles can be adjusted to a minimum based on eitherthe known distance between the vehicle and the control receiver, witheach vehicle transmitting just enough power so that detection of thesignal by the control station is assured. A further or alternative poweradjustment may be made by the vehicle transmitter based on the powerreceived from the control station, for example, during the query.Finally, a closed loop system in which the query includes instructionsas to the power levels to be used may be used. It is not desired thatsuch closed loop system result in exactly the same power level beingreceived from each remote station be perfect since this would increasethe probability of amplitude correlation between the signals andresultant destructive interference, usually in a situation where astrong line of sight transmission exists between a few vehicles and thebase station. A balance should be struck between a reduced variation inthe power level received by the control center from the various remotevehicles and keeping the chances of destructive interference low.

[0209] Increased pulse duration can also reduce the transmitted powerfor a given ratio of detection probability to false alarm probabilityespecially in the receiver shown in FIG. 15 and described below.

[0210] Preferably, the amplitude of the signals broadcast during thetime slot is shaped over the broadcast period to reduce the side-lobesof the signals and avoid false signals in adjacent frequency slots,which may be a problem when large numbers of vehicles broadcast at thesame time. Alternatively or additionally, frequency windowing at thereceiver may be used to reduce cross-talk between channels.

[0211] Dynamic range limitations can be reduced by providing multiplereceivers, each covering only a portion of the frequency band. Finallythe novel receiver of FIG. 15 may be used to determine the presence orabsence of signals in particular slots.

[0212]FIG. 15 shows a receiver system corresponding generally toreference number 92 and to a portion of computer 80 of FIG. 14. Ingeneral such a receiver is also useful for the first phase of thedispatching system described above as well as for the IVHS system.

[0213] An antenna 94 (or an array of antennas) receives signals from aplurality of vehicles simultaneously and passes them to receiver and(optionally) AGC 96. Receiver and AGC 96, which may be of conventionaldesign, downconverts the received signals from RF to IF frequencies. Thethreshold levels of the detection process may be dependent on the AGCprocess. The IF signal is digitized by an A/D system 98 and further downconverted by a downconverter 100 to base band. It should be understoodthat this receiver/downconverter system does not demodulate the incomingsignals, but only downconverts the RF so that the same relativefrequency differences of the signals is present at the output ofconvertor 100 as in the incoming signals, except that the absolutefrequency has been reduced to a low frequency from the RF frequency ofthe transmitted signal. At these lower frequencies digital systems canbe used to analyze and detect the signals.

[0214] The low frequency band signals are fed to a series of correlationfilters 102 (correlation-type receiver), each of which has a very narrowbandwidth which is related to the correlation time of the correlationfilter. Preferably, the frequency bandwidths of adjacent receivers 102overlap so that the entire bandwidth of each of the slots is covered byone set of receivers 102. The output of each of the receivers iscompared to a threshold 104 to determine if a signal is present at thefrequency of the respective receiver 102 and the outputs of all ofthreshold detectors for a given slot are OR gated to determine if anysignal is present in the slot. Alternatively, the outputs of thecorrelation receivers can be summed and this sum signal used todetermine if any signal is present in the slot. However, this willgenerally result in increased noise.

[0215] In an alternative preferred embodiment of the invention, thestrongest output of the set of correlation receivers is chosen forcomparison with a threshold, with or without post-detection integration.

[0216] Use of a plurality of overlapping narrow band receivers in thismanner also reduces the extent of side lobes of the detection processoutside the band of the slot. This allows for closer frequency spacingof the slots since interference between slots having adjacentfrequencies is reduced.

[0217] One set of receivers 102, threshold detectors 104 and an OR gateis provided for each slot and is referred to herein as a slot detectorunit. Slot detector units for all of the slots feed a data processor 108which, together with computer 80 processes the data as described above.When large numbers of vehicles are used in the system andintermodulation becomes a problem (or if AGC is used, and low levelsignals are lost), it may be necessary to provide a plurality of frontend portions of receiver 92 (the front end being defined as receiver 96,convertor 98 and converter 100), where each front end receives signalsfrom only a portion of the entire frequency band including one or manyof the slots. The function of correlation receivers 102 may also beimplemented, for example, using set of DFTs or an FFT (for CW signals),matched filters or other correlation receiver methods or other optimumreceiver methods, depending on the transmitted signals. Other methodssuch as energy detectors (e.g., radiometers) with or without trackingmay also be used, however, they will give less optimal results, becauseof practical limitations on input band-pass filter designs.

[0218] It should be understood that using a plurality of correlationreceivers for the same slot may increase the false alarm probability andhence the threshold for positive detection may be adjusted to provide adesired low false alarm probability.

[0219] For all the above applications of the invention, destructivesignal interference between units which broadcast in the same slot canbe further reduced by performing transmission diversity operations suchas random phase (for example, 180° and 0°) and/or amplitude changes, atthe transmitters of the remote stations.

[0220] Additionally or alternatively, the effect of interference fromlarge signals on small nearby signals can be reduced by performingdetection in a two step process. In the first step, only slots havingsignals having a value above a given level are validated. This level canbe fixed in advance or can be adaptive, depending on the signal levelsactually detected. If none of the signals are high enough to causeconcern that they have caused signals to occur in other slots then,preferably, there is no need for the second step and all signals above abaseline level are validated.

[0221] If the two step detection process is chosen, then all stations,except those broadcasting at the validated slots are asked to broadcastagain. This will avoid spillover into other slots and the smallersignals can then be properly received without interference. It may benecessary in some cases to repeat this process additional times if verylarge signal variations are expected.

[0222] In a preferred embodiment of the invention, an improvedprobability of detection may be desired for some of the slots, such as,for example, the control slots. For these slots repeated transmission ofsignals using transmission diversity may be performed and detectionenhancement methods such as post detection integration may be used toimprove the detection probability.

[0223] The system may also be provided with a display 110 for displayingthe data, such as the maps and graphs of FIGS. 10-13 and with a userinterface 112 which is used by an operator to control both the operationof the system. The user interface also preferably controls the displayand the memory to allow for the operator to review the maps previouslygenerated or to generated new displays based on information previouslyreceived.

[0224] Information may be sent by the control center to the vehicles toenable them to minimize average travel delays. This information mayconsist of the above mentioned maps or of travel delay information atvarious intersections. The vehicles can then use this information tooptimize their route. Alternatively, the control center may send routinginformation to some of the vehicles in order to equalize traffic delays.In either event, the fast response of the system in a matter of secondsallows for real time supervision, adjustment and continuousstabilization of traffic patterns with additional iterations. Asdescribed above, in a distributed system only prospective trafficpatterns is broadcast by the control center and each vehicle calculatesits own route.

[0225] The IVHS system described above is also useful in trackingsituations such as for fleet management.

[0226] In a further preferred embodiment of the invention the positionand other characteristics of a large number of vehicles can be mappedand tracked in near real time using a relatively narrow bandwidth. Inthis embodiment each vehicle is assigned a number of slots, which areused only by that vehicle, according to a predetermined protocol.

[0227] The vehicles are preferably first mapped with a preferred mappingphase of a mapping and tracking procedure. A way to perform this phaseis to devote a small matrix to each vehicle to be tracked. This smallermatrix is part of the entire matrix of slots assigned by the controlcenter. The smaller matrix represents, for example, (in mapping ofspatial coordinates) a square area which is divided into nine sub-areas,each of the sub-areas represented by one of nine slots assigned to aparticular vehicle. In a first iteration a large area is divided intonine sub-areas and a vehicle broadcasts a signal in the slot whichcorresponds to its present position. In a second step of the mappingphase, the area in which the vehicle previously broadcast is expanded tofill the nine slots, with a finer resolution. Alternatively, the areawhich is zoomed into the nine slots is slightly larger than the area ofthe previous broadcast to avoid a situation in which the vehicle was atthe border of the area and left the area between steps.

[0228] This identification of one sub-area and consequent convergence toa higher resolution is repeated several times until the requiredresolution is achieved. The highest practical resolution, as will becomeclear below, is the distance that a vehicle could travel in the time ittakes to perform a tracking cycle as described below. Within fiveiterations the individual resolution can be improved from a 10 km square(divided into nine 3.3 km squares) to resolution of about a 40 metersquare area.

[0229] It should be understood that while this aspect of the inventionis generally described with respect to a two dimensional spatial matrix(north-south and east-west for example) the invention is especiallyuseful and efficient for tracking buses, trains or other such movingremote stations which move along a line (their route). In this verycommon situation, only one dimensional positional information isrequired, the dimension being the distance along the route.

[0230] In a second, tracking, phase of the mapping and trackingprocedure, performed periodically after the required resolution isreached, nine slots, for example, representing a 3×3 area of minimumresolution areas, are used to track additional movements of the vehicle.In one embodiment of the invention the central one of the nine areascorresponds to the area occupied by the vehicle at the end of themapping phase (or during a previous periodic updating iteration of thetracking phase). During each periodic update, each vehicle broadcasts ina slot which corresponds to either its previous position (the slotcorresponding to the center area of the 3×3 group of areas) or one ofthe adjoining areas. In the next iteration, the newly chosen area is thecenter of the 3×3 matrix, according to a predetermined protocol.

[0231] For a one-dimensional tracking system, only three slots arerequired, where one slot (conceptually the center slot) represents thelast previous position along the route and the other two slots representpositions in the two directions along the route.

[0232] In a further preferred variation of this embodiment of theinvention, only 5 slots are utilized to map into the 3×3 area. One ofthese slots represents, as a reference, one of the corner (or thecenter) areas of the 3×3 area and the other 4 slots representnorth-south or east-west variations. In this the vehicle may broadcastduring one or two of the five slots, depending on the deviation, if any,from the corner (or center) area chosen as reference. If the vehicle isin the reference area, broadcasting takes place only in the slot whichrepresents the reference area. If the vehicle is in the areasnorth-south-east or west of the reference, then the vehicle broadcastsin only one slot representing such deviation from the reference. If thevehicle moves into an area diagonally shifted from the reference, thevehicle will broadcast during two slots representing, for example, theeast-west and north-south deviations of the area in which the vehicle issituated.

[0233] Similarly, only two slots would be needed for one-dimensionalmapping.

[0234] If a vehicle does not respond or its response was not detectedduring a given iteration (i.e., if the vehicle is lost or an erroneouscode is received) a number of remedial steps are possible. Theparticular vehicle (or some or all the vehicles) may be requested toretransmit the particular iteration, or may be asked to return toperform the previous iteration or a sequence of previous iterations orto operate at a lower mapping resolution. In some situations it may bedesirable to start the process over, at the lowest resolution for theparticular vehicle or for all of the vehicles, for example to repeat theentire process or some of the steps of the process to increasereliability and to deduce the extent of accumulated errors, even if noerrors are detected. The process may be repeated using the previousposition information or with present information with a lower resolutionespecially to find “lost” vehicles.

[0235]FIG. 16B shows the slots in which a signal would be broadcast inthe tracking phase (or possibly in the mapping phase) to indicate eachof three positions of the vehicle shown in FIG. 16A, while FIG. 16Cshows the slots which would be used if a corner area were used as thereference. It should be noted that for either case the vehicle was inthe center area during the previous tracking iteration.

[0236] While it appears from FIGS. 16A-16C that it is desirable to havenorth south deviations represented by a change in slot frequency andeast west variations represented by a change in slot time, it isactually more practical to use the same frequency for all small matrixslots used by a particular vehicle, since this requires only onetransmitter per vehicle.

[0237] While it is desirable to dedicate particular transmission slotsfor each vehicle, it is possible to have overlapping assignedtransmission slots. For example, if one slot for one vehicle is the sameas a slot for another vehicle, then if a signal is received in theshared slot, the systems checks if a signal was received in one of theunshared slots for one of the vehicles. If it was, then the signal inthe shared slot is considered as coming from the other vehicle. If nosignal is received in unshared slots for either vehicle, then the signalis considered as coming from both vehicles.

[0238] In a preferred embodiment of the invention, nine areas arerepresented by a four bit word which is more than sufficient to definethe 3×3 matrix of elements. In this or similar cases the “physicalslots” described above may be represented by any convenient code.

[0239] Furthermore, the logical slots may have different meanings orresolutions in the same logical matrices. For example, the positionresolution of the logical elements may depend on the maximum expectedvelocity of the vehicle and the resolution in the two mapped directionsneed not be the same. Additionally, for mapping and tracking along aroad, one dimension may be the (one dimensional) position along thelength of the road and other logical slots, if any, may, for example,represent the lane in which the vehicle is traveling. Alternatively, asdescribed above, only one-dimensional tracking may be performed.

[0240] While according to one preferred embodiment of the invention, asdescribed in the previous paragraph, the spatial resolution of thesystem is fixed and depend on the maximum expected velocity of thesystem, this embodiment limits the number of vehicles which can betracked and/or the spatial resolution with which they are tracked. Forexample, if the time between queries is three seconds, the spaceresolution cannot be any finer than the distance the vehicle wouldtravel at maximum speed. If a particular resolution is required for slowmoving vehicles, then the number of vehicles must be limited so that novehicle, moving at its top speed, would be outside the range of the 3×3or 3×1 position matrix at this resolution when the next position queryis responded to.

[0241] In order to improve the trade-off between spatial resolution andnumber of vehicles, the resolution of the system is adapted to thecurrent speed of the vehicle, in accordance with a preferred embodimentof the invention.

[0242] In accordance with one particular embodiment, an additional slotis associated with each vehicle. The vehicle transmits a signal in thisslot in accordance with its present velocity (or distance traveled sincethe previous query) and an associated resolution of the slot. Thus ifthe vehicle velocity (or distance traveled) is greater than a givenvelocity (or distance), a signal is broadcast in the additional slot.When such a signal is broadcast, the resolution of the slots isdecreased to accommodate the higher velocity (or distance). When no suchsignal is broadcast, the resolution is at the higher level.

[0243] In accordance with a second particular embodiment, no additionalslot is required for adaptive resolution. In addition, the resolutionmay be varied in small discreet steps rather than there being only twosteps. In accordance with this embodiment, the resolution represented bythe slots depends on the prior history of the vehicle. If, for example,if a vehicle broadcasts that it is moving in a particular direction formore than a given number of iterative queries of position, thesupposition is that its speed is increasing. For this situation theresolution of the system, for that vehicle, is reduced, automatically,by increasing the distance represented by each resolution element. Forexample, if a bus or other unit broadcasts twice (or alternatively threetimes) in a row in the slot representing movement out of the centralslot in a given direction, the resolution is decreased by a givenpercentage, for example 10%-50%. If the situation continues, then theresolution is decreased further, until the unit broadcasts, at leastsome of the time, in the slot which represents the “central” position.On the other hand, if the unit responds in the central position morethan a given number of times, then supposition is that the speed isdecreasing or that the unit is stopped. In this situation the resolutionis increased, in stages, until either the unit broadcasts occasionallyin the left of right positions or until it reaches a maximum resolution.

[0244] It should be understood that since the unit does not generallyaccelerate to high speed in a very short time, especially from a stop,there is little or no chance that synchronization will be lost, even ifa very high resolution is used when the vehicle is stopped or movingvery slowly. The highest resolution may, in this embodiment, be limitedonly by how far the vehicle can move from a stopped condition, duringthe system cycle time. This is a much higher resolution than thedistance it can move at top speed which is the highest resolution forthe non-adaptive system.

[0245] In accordance with a third particular embodiment, a 4×4 or 4×1matrix of positions is allocated, depending on whether two or threedimensional mapping is required. In this embodiment, the center of theposition element which was previously reported as containing the unit istranslated, for the next iteration, to reside at the center of thematrix, i.e., between resolution elements. The distances represented bythe slots is then adjusted based on the history of position indicationsreceived from the unit.

[0246] In accordance with a fourth particular embodiment of theinvention a 2×2 or 2×1 matrix is assigned for each vehicle during thetracking stage. The central position represents the center of the lastprevious reported position element as in the third particularembodiment. In this system, the unit must report “movement” to the leftor right. However, this reporting actually represents whether it is atthe left or right of the center of the last previous reporting elementand may not represent actual movements to the left or right or anyactual movement at all.

[0247] If the vehicle reports that it has moved to the left during thequery time for two (or three) consecutive queries, then the distancerepresented by each slot is increased, as described above. When theresolution is consistent with the speed of the unit, the unit willreport alternate leftward and rightward “movements.” It should beunderstood that this reporting does not represent actual leftward andrightward movements but rather movements greater than or less than onehalf-resolution element.

[0248] If the resolution has been decreased as a result of movement inone direction and the resolution has been adjusted to suit, partially orfully, continued alternating leftward and rightward reports mayrepresent either continued movement at the same speed or slowing down ofthe vehicle. Thus, the size of the resolution element is reduced (theresolution is increased) until the alternative direction reportingsequence no longer holds or until a minimum resolution element isachieved. It should be understood, that when the unit is stationary, thealternative reporting situation will always result, so that an increasein resolution (to the maximum available based on the acceleration of theunit) should be attempted whenever the alternative direction reportingsequence is achieved.

[0249] In accordance with a fifth particular embodiment, utilizing thesame logic as the fourth embodiment, only a single slot need be assignedto each vehicle traveling along a route. In this fifth embodiment thebroadcasting of a signal within the slot or silence indicates either theleft or right logical slot of the fourth particular embodiment.

[0250] Furthermore, if only forward movement is assumed, then at leastone less slot is required (except of course for the fifth particularembodiment, since movement need only be reported in one direction foreach dimension.

[0251] In addition to tracking position, the above described systems canbe used to report additional variables. For example, for buses, otherslots may be used to provide other information about the bus such asoccupancy level. In such a situation after an initialization phasesimilar to that for position, one slot may represent no substantialchange in the occupancy, with two other slots representing an increasedoccupancy of the bus and another slot representing a decreasedoccupancy. Such allocation allows for occupancy to be tracked in asimple way, similar to the position tracking described above,simultaneously with the position tracking or at a lower frequency,interleaved with the position tracking responses.

[0252] While the preceding mapping and tracking system implies that eachvehicle in the system is a priori included in the map, this is notnecessarily a requirement of the system. Each of these vehicles wouldthen be assigned slots for use in the mapping and/or tracking phases.The vehicles from whom identification is requested may be chosen inaccordance with a criteria of the vehicle determined according to anyone of the procedures outlined above in which vehicle having certaincharacteristics are determined.

[0253] In a mapping and/or tracking system according to the presentinvention only a small number of bits must be transmitted for eachiteration. Using such a system can be especially worthwhile if thetransmission protocols and equipment are designed according to thecriteria described above with regard to the targeting and identificationand IVHS systems. In the present system, and for systems having onlyseveral bits (such as up to 10 or a few tens of bits) of information, itis useful to use a transmission protocol without interleaving and FEC.Furthermore, protocols which require a preamble including a substantialnumber of training pulses for locking onto the frequency are alsowasteful in view of the small number of transmitted pulses. In apreferred transmission/receiving system, the transmitted pulses are madelong enough so that their bandwidth is very narrow and are received by asystem which is capable of taking advantage of such a narrow bandwidthsystem despite the inherent instability of the transmitters. Such asystem is described above with respect to FIG. 15.

[0254] Furthermore, the transmission power of such systems need not bevery high, even for transmission over relatively long distances, becauseof the very high effective signal to noise ratio of the receivers. Nordoes the power have to be very constant over the transmission, since thereceiver is sensitive to the total energy in the pulses and is notsensitive to the transient rise of the transmitter. Thus, there is noneed in the present system to wait for the power in the transmission torise to the design value as in conventional systems. This combination offactors allows for transmission to begin almost immediately, withoutconsidering the attack, or rise time of the transmitter.

[0255] It may also be useful, in a preferred embodiment of theinvention, to provide diversity techniques, such as time and/orfrequency and/or microscopic space diversity techniques, as are known inthe art, to improve system reliability, especially in a mobilecommunication environment.

[0256] Thus, while the pulse widths used in the present system may beseveral times longer than those used in standard digital transmissionsystems, the lack of overhead caused by the absence of lockingpreambles, wait for transmitter power attack time and error correctionmore than makes up for the longer pulse times, for relatively smallnumbers of bits per transmission. The exact design of a system dependson many factors such as distance, power available, resolution required,available bandwidth, etc. However, it has been found that, in general,the present invention provides a much higher capability for mapping andtracking than conventional systems.

[0257] In general, one or more base stations may be used forbroadcasting calls and/or receiving responses from remote stations. Ifmore than one base station is used, each station preferably performs areduction of the data which it receives by either choosing its bestcandidate for performing the task or by performing a mapping function ofits nearby region or of its associated vehicles. The base stations thenpreferably send this reduced information to a central base station whichmakes the final decision, constructs the desired map or performs anyother final analysis. Furthermore, the central base station would, in apreferred embodiment of the invention, instruct each of the basestations as to which additional queries they should make. In thissituation the subsequent queries need not be the same for all the basestations.

[0258] In addition, in a preferred embodiment of the systems of theinvention, the base station broadcasts the information which it hasreceived from all the stations. This information is preferably broadcaston a separate conventional data transmission channel. The signal isreceived by the remote stations and is used for error correction by themand, preferably, to allow for improved stabilization of a trafficsituation or improved interaction between the various remote vehicles asdescribed above.

[0259] For large areas of coverage, the area may be serviced by aplurality of base stations which are all available to receive signalsfrom all of the remote stations. This redundancy of base stationreceivers allows for mapping and tracking over a larger area than with asingle base station. Furthermore, in a preferred embodiment of theinvention, the base stations are the preferential receiver for thoseremote stations which are closest to it. In this regard, transfer of aremote station from one preferential base station to another isautomatic since it can be made based on the previous map of thepositions of the remote stations. Such preferential assignment of theremote stations to a base station may be accomplished without any actionby the remote station such as a change in slot allocation and thechances of losing synchronization with the remote station, as oftenoccurs with cellular systems, is minimized.

[0260] While the preferred base station is the primary receiver ofsignals from “its” remote stations, signals received from other basestations may also be used. If such signals are used, the weight given tothem may depend on the position of the remote station. This may beconsidered an advanced form of macroscopic diversity.

[0261] In a preferred embodiment of the invention, the targeting andpreferably the identification protocols described above may be performedin conjunction with the mapping and tracking protocols of the presentinvention. In this way a subset of remote stations whose movement orother characteristic values are of interest are first targeted,preferably identified and then mapped and cracked in accordance withmapping and targeting protocols. It may be desirable to assignparticular slots to the identified units prior to the mapping andtracking sequences.

[0262] If the targeting criterion is the same as the criteria to betracked (for example position), then the mapping stage may begin at theresolution range utilized in the targeting phase, or omitted altogether.

[0263] In a further preferred embodiment of the invention, a pagersystem having an appointment making capability operates on principlessimilar to those described above. For example, the pager system maybroadcast a request to one or more pagers which also incorporate anappointment calendar. The individual pagers broadcast a signal in one ormore of a matrix of slots which correspond to busy times. Theappointment may then be made for other times. The individuals are thennotified, by pager, that an appointment has been made for them.

[0264] In many of the above embodiments of the invention, the system istriggered and/or synchronized according to a synch signal broadcast bythe control station. Other sources of synchronization, which synchronizeboth the remote and control station, such as GPS received signals orother timing signals, can be used to trigger and/or synchronize thesystem.

[0265] Reference is made to PCT application PCT/EP95/01330, thedisclosure of which is incorporated herein by reference, and especiallyto an analysis of the communication resources required by some of theembodiments of the above described inventive apparatus and methodcompared to polling. This analysis may be useful in giving a betterunderstanding of the reasons for the improved performance of the presentinvention.

[0266] The invention has been described herein using examples in whichthe indication signals are transmitted in time, frequency or time andfrequency slots. Other types of transmission slots are also useful inthe invention such as frequency hopping and other spread-spectrumtransmission slots. The term “transmission slots” or “slots” as usedherein includes all these types of slots.

1. A method of mapping a plurality of remote stations each having avarying attribute affecting a characteristic value computed according toa predetermined procedure comprising: (a) assigning a plurality oftransmission slots to each of the remote stations; (b) determining, bythe respective stations, of their characteristic values; (c) initiallybroadcasting, by the respective stations, of their determinedcharacteristic values in said plurality of transmission slots, saidbroadcast characteristic value having a first characteristic valueresolution; and (d) subsequently broadcasting, by the stations, of theirrespective characteristic values in said plurality of transmissionslots, said subsequent broadcasting having a finer characteristic valueresolution relative to said previously broadcasted characteristic valuehaving a first characteristic value resolution.
 2. A method of mappingaccording to claim 1 comprising repeating (d) with successively finercharacteristic value resolutions until the characteristic value isbroadcast with a desired characteristic value resolution.
 3. A method ofmapping according to claim 1 or claim 2 wherein the finer characteristicvalue resolution is twice as fine as that of the previous characteristicvalue resolution.
 4. A method of mapping according to claim 1 or claim 2wherein the finer characteristic value resolution is less than twice asfine as that of the first characteristic value resolution.
 5. A methodof mapping according to claim 1 or claim 2 wherein a characteristicvalue mapping space is divided into a fixed number of portions andwherein said initial broadcast indicates which of said portions containsthe characteristic value.
 6. A method of mapping according to claim 5wherein said initially broadcast portion is divided into a fixed numberof portions of a smaller size and wherein said subsequent broadcastindicates which of said portions of smaller size contains thecharacteristic value.
 7. A method of mapping according to claim 5wherein an area somewhat larger than said initially broadcast portion isdivided into a fixed number of portions of a smaller size and whereinsaid subsequent broadcast indicates which of said portions of smallersize contains the characteristic value.
 8. A method of mapping accordingto claim 1 or claim 2 and including repeating at least one step ofbroadcasting, at a coarser characteristic value resolution, when aninvalid signal is received from a remote station.
 9. A method of mappingaccording to claim 1 or claim 2 wherein at least one step ofbroadcasting is repeated periodically to reduce accumulated errors inthe characteristic value.
 10. A method of mapping according to claim 1or claim 2 wherein the characteristic value is the location of a mobileremote station.
 11. A method of mapping according to claim 1 or claim 2in which a base station receives the broadcast signals and transmits adata stream including information regarding the broadcast signals asreceived by the base station.
 12. A method of mapping according to claim11 in which the remote stations receive the transmitted data stream andutilize it to determine if their signals have been correctly received bythe base station.
 13. A method of mapping according to claim 1 or claim2 in which a plurality of base stations receive signals broadcast by theremote stations.
 14. A method of mapping according to claim 1 or claim 2in which the plurality of remote stations is targeted from a largernumber of remote stations based on a particular criterion.
 15. A methodof mapping according to claim 14 wherein the particular criterion is thevalue of the characteristic value possessed by the remote station.
 16. Amethod of tracking a characteristic value of a plurality of remotestations each having a varying attribute affecting the characteristicvalue computed according to a predetermined procedure, comprising: (a)assigning a plurality of transmission slots to each of the remotestations; (b) determining, by the respective stations, of theircharacteristic values locations relative to a previously determinedcharacteristic values; and (c) broadcasting, by the respective stations,of their determined characteristic values in said plurality oftransmission slots, relative to the previously determined characteristicvalue.
 17. A method of tracking according to claim 16, comprisingiteratively repeating (b) and (c) wherein said previously determinedcharacteristic value is the characteristic value determined in theprevious iteration.
 18. A method of tracking according to claim 17wherein a characteristic value region surrounding said previouslydetermined characteristic value is divided into a plurality ofcontiguous regions and wherein the relative characteristic value whichis broadcast comprises broadcasting a signal in one or more of thetransmission slots which indicates which of the regions contains thedetermined characteristic value.
 19. A method of tracking according toclaim 18 wherein the extent of the surrounding regions is establishedbased on an expected maximum rate of change in the characteristic valueof the remote station.
 20. A method of tracking according to claim 18wherein the extent of the surrounding regions is established based onthe estimated actual rate of change of the characteristic value of theremote station.
 21. A method of tracking according to claim 20 whereinthe remote station transmits a signal in an additional slot indicatingthe resolution level.
 22. A method of tracking according to claim 20wherein the remote station utilizes one of a plurality of resolutionlevels depending on the rate of change of the characteristic value andwherein the resolution level utilized depends on the history of thebroadcasts of movement of the vehicle.
 23. A method of trackingaccording claim 22 wherein the size of the resolution element isincreased when a remote station broadcasts a change in value in a givensense for a predetermined number of consecutive broadcasts.
 24. A methodof tracking according to claim 22 wherein the size of the resolutionelement is decreased when a remote station broadcasts no change incharacteristic value for a predetermined number of consecutivebroadcasts.
 25. A method of tracking according to claim 23 wherein thesize of the resolution element is decreased when a remote stationbroadcasts no change in characteristic value for a predetermined numberof consecutive broadcasts.
 26. A method of tracking according to any ofclaims 16-25 wherein the characteristic value is a vector of dimensionn, and wherein (3)^(n) slots are allocated for broadcasting thecharacteristic value, n being greater than or equal to
 1. 27. A methodof tracking according to any of claims 16-25 wherein the characteristicvalue is a vector of dimension n, and wherein 2n+1 slots are allocatedfor broadcasting the characteristic value, n being greater than or equalto
 1. 28. A method of tracking according to any of claims 16-23 whereinthe characteristic value is a vector of dimension n, and wherein 2^(n)slots are allocated for broadcasting the characteristic value, n beinggreater than or equal to
 1. 29. A method of tracking according to any ofclaims 16-23 wherein the characteristic value is a vector of dimensionn, and wherein n+1 slots are allocated for broadcasting thecharacteristic value, n being greater than or equal to
 1. 30. A methodof tracking according to any of claims 16-23 wherein the characteristicvalue is a vector of dimension n which can increase or decrease, andwherein one slot is allocated for broadcasting the characteristic value,n being greater than or equal to
 1. 31. A method of tracking accordingto claim 30 wherein the presence of a broadcast signal indicates adeviation in one sense from the previously broadcast characteristicvalue and the absence of a broadcast signal indicates a deviation in theopposite sense from the previously broadcast value.
 32. A method oftracking according to any of claims 16-23 wherein the characteristicvalue is a vector of dimension n which varies in only one sense andwherein (2)^(n) slots are allocated for broadcasting the characteristicvalue, n being greater than or equal to
 1. 33. A method of trackingaccording to any of claims 16-23 wherein the characteristic value is avector of dimension n which varies in only one sense and wherein n+1slots are allocated for broadcasting the characteristic value, n beinggreater than or equal to
 1. 34. A method of tracking according to any ofclaims 20-22 wherein the characteristic value is a vector of dimension nwhich varies in only one sense and wherein only one slot is allocatedfor broadcasting the characteristic value, n being greater than or equalto
 1. 35. A method of tracking to any of claims 16-25 and includingrepeating at least one step of broadcasting, at a coarser characteristicvalue resolution, when a valid signal is not received from a remotestation during tracking.
 36. A method of tracking according to any ofclaims 16-25 wherein at least one step of broadcasting is repeatedperiodically to reduce accumulated errors in the characteristic value.37. A method of tracking according to any of claims 16-25 wherein thecharacteristic value is the location of a mobile remote station.
 38. Amethod of tracking according to any of claims 16-25 in which a basestation receives the broadcast signals and transmits a data streamincluding information regarding the broadcast signals as received by thebase station.
 39. A method of tracking according to claim 38 in whichthe remote stations receive the transmitted data stream and utilize itto determine if their signals have been correctly received by the basestation.
 40. A method of tracking according to any of claims 16-25including a plurality of base stations which receive signals broadcastby the remote stations.
 41. A method of tracking according to any ofclaims 16-25 wherein the previously determined characteristic value isdetermined in accordance with claim 1 or claim 2.